Organic Chemistry.

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  • Created by: Sophhoyle
  • Created on: 21-03-16 12:04

Representing Organic Compounds.

Type of Formula

General Formula- An algebraic formula that can describe any member of a family of compounds. E.g. CnH2n
Empirical Formula- The simplest ratio of atoms of each element in a compound. E.g. Ethane (C2Hhas an empirical formula of CH3.)

Molecular Formula- The actual number of atoms of each element in a molecule.

Structural Formula- Shows the atoms carbon by carbon, with the attached hydrogen and functional groups. E.g. CH3CH2CH2CH2OH

Skeletal Formula- Shows the bonds of the carbon skeleton only, with any functional groups.

Displayed Formula- Shows how all the atoms are arranged, and the bonds between them.

(http://adashofscience.com/wp-content/uploads/sites/3/2013/04/Isobutanol-skeletal.png)(http://www.bbc.co.uk/staticarchive/71083413523051d66d39fd1fbcf8d1ec381abc23.gif)

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Homologous Compounds And Series.

Homologous Series- Is a group of compounds that contain the same functional group. They all have the same general formula.

Each successive member of a homologous series differs by a CH2 group.

Homologous Series And Its Prefix

  • Alkanes have a prefix of -ane.
  • Branched Alkanes have a prefix of -alkyl.
  • Alkenes have a prefix of -ene.
  • Halogenoalkanes have a prefix of fluoro-/chloro-/bromo-/iodo-.
  • Alcohols have a prefix of -ol.
  • Aldehydes have a prefix of -al.
  • Ketones have a prefix of -one.
  • Cycloalkanes have a prefix of cyclo- -ane.
  • Carboxylic Acids have a prefix of -oic acid.
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Nomenclature.

LUPAC Rules

1. Count the carbons in the longest continous chain that contains the functional group- giving you the stem:

  • 1 carbon- Meth-             2. The main functional group of the molecule usually gives 
  • 2 carbon- Eth-                    you the end (suffix)..See table on prior card. 
  • 3 carbon- Prop-             3. Number the carbons in the longest carbon chain so that 
  • 4 carbon- But-                   the carbon with the functional group attached has the    
  • 5 carbon- Pent-                 lowest number.
  • 6 carbon- Hex-              4. Write the carbon number that the functional group is on                                                before the suffix.

5. Any side chains or less important functional groups are added as prefixes at start of the name. Put them alphabetical with the no. of the carbon the functional group is attached to.

6. If there's more than one identical side- chain or functional group, use di- (2), tri- (3) or tetra- (4). 

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Basic Mechanism.

  • Scientists often want to know how a reaction happens, rather than just knowing the product.
  • Mechanisms can be used to break reactions down into individual stages.
  • They show how molecules react by using curly arrows to show the bonds made and broken.
  • A curly arrow shows where a pair of electors goes during a reaction.
  • The arrow starts at the beginning of the reaction where the lone pair of electrons are and it ends where the new bond is formed.(http://chemwiki.ucdavis.edu/@api/deki/files/5297/=image005.png?revision=1)
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Isomers Intro. And Structural Isomers.

Isomers- two molecules that have the same molecular formula, but the atoms are arranged differently.

Structural Isomers

Chain Isomers: They have different arrangements of the carbon skeleton.

Positional Isomers: They have the same skeleton and the same atoms/groups attached, but they atom or group of is attached to a different carbon.

Functional Group Isomers: They have the same atoms arranged into different functional groups.

(http://images.tutorcircle.com/cms/images/39/chain-isomerism.png)(http://www.scienceaid.co.uk/chemistry/organic/images/haloisomers.jpg)

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Stereoisomers

Stereoisomers- have the same structural formula but a different arrangment.

  • Double bonds can't rotate.
  • Carbon atoms in a carbon:carbon double bond and the atoms bonded to these carbons all lie in the same plane.
  • Another important thing about carbon:carbon double bonds is that atoms cant rotate around them like they can around single bonds. In fact, double bonds are fairly ridgid and do not bend much.
  • The restricted rotation around the carbon:carbon double bond causes a type of stereoisomerism called E/Z isomerism.
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E/Z Isomerism.

  • Alkenes have restricted rotation around their carbon:carbon double bonds.
  • This means that if both bond carbons have different atoms/groups attached to them, the arrangement of those groups becomes important. And you end up with two stereoisomers.
  • These are: E-isomer and Z-isomer.
  • The Z-isomer has the same groups either both above or both below the double bond.
  • The E-isomer has the same groups positioned across the double bond.(http://chemwiki.ucdavis.edu/@api/deki/files/4913/=image157.png?revision=1&size=bestfit&width=285&height=95)
  • A molecule which has a carbon:carbon double bond surrounded by four different groups still has an E and Z isomer, it is just harder to work out. However, it can be worked out by using Cahn-Ingold-Prelog (CIP).
  • The first step is to label the two carbons 1 and 2.
  • Secondly, you must look at the atoms bonded to each carbon, the one with the higher atomic number on each carbon is given the higher priority.
  • Lastly, the two higher priority atoms are given the E or Z isomerism.
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Alkanes Basics.

  • Alkanes have the general formula: CnH2n+2.
  • They only contain carbon and hydrogen, so are hydrocarbons.
  • Every carbon atom in an alkane has four single bonds with other atoms. It is imposible for carbon to make more than four bonds.
  • This means that alkanes are saturated (only contain single bonds.)
  • You can get cycloalkanes too. They are a ring of carbon atoms with two hydrogen atoms attached to each one.
  • Cycloalkanes have a different general formula from that of normal alkanes (CnH2n), but they are still saturated.

(http://4.bp.blogspot.com/-SyHaym-_63Y/UQsYeRKF3cI/AAAAAAAAA20/OSNshYOCB3A/s1600/alkane-02.png)(http://chemistry2.csudh.edu/rpendarvis/cyhexRep.GIF)

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Alkanes Continued...Crude Oil.

Petroleum- Crude Oil.

  • Petroleum is a mixture of hydrocarbons. It is mostly made up of alkanes, small to large.
  • Crude oil can be seperated into fractions, using fractional distilation.

How it works.

  • First the crude oil is vaporised at about 350 degrees celsius.
  • The vapourised oil goes into a fractionating column and rises through the trays. The largest hydrocarbons do not vaporise as their boiling points are too high. Instead, they just form a residue at the bottom of the column.
  • As the vapour moves up the fractioning column it decreases in temperature. Due to each hydrocarbons differing chain length, they have different boiling points, so each fraction condenses at a different temperature. At each point, they are drawn off.
  • The hydrocarbons with the lowest boiling points are drawn of as gasses at the top of the column.
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Alkanes Continued...Cracking.

  • Many industries want the light fractions of crude oil, including: petrol and naphtha. 
  • To meet this demand, they use cracking. Cracking is the process used to break long-chain alkanes into smaller hydrocarbons.

(http://www.larapedia.com/chemistry_notes/organic_chemistry_clip_image029_0000.png)

  • There are two types of cracking to note, thermal and catalytic.
  • Thermal cracking takes place at high temperature (up to 1000 celsius) and high pressure (up to 70 atm.) It produces a lot of alkenes. And can be used to make valuable polymers, including polyethene.
  • Catalytic cracking uses a zeolite catalyst (hydrated aluminosilicate), at a slight pressure and a high temperature (450 celsius.) It is used to produce motor fuels and can be done at low cost, due to the catalyst.
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Alkanes As Fuels.

  • If you burn (oxidise) alkanes with plenty of oxygen, you get carbon dioxide and water. This is a condensation reaction.
  • Complete combustion is when the only products are carbon dioxide and water.
  • Alkanes make a great fuel, burning a small amount releases a lot of energy.
  • However, the down side of this is the pollution it causes.
  • Incomplete combustion occurs when there is not enough oxygen for complete combustion to occur. This changes the products of a reaction and can cause nasty side effects.
  • Carbon monoxide can be created in incomplete combustion. This is an issue as it is poisonous. It also is an issue as carbon monoxide binds to haemoglobin molecules in red blood cells as oxygen molecules. So oxygen can be carried around the body. Luckily, it can be removed from exhaust gases by catalytic converters in cars.
  • Carbon particles (soot) can be created in incomplete combustion. This is an issue as it can cause breathing problems, aswell as building up in engines, so they dont work properly.
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Alkanes As Fuels Part 2.

  • Burning fossil fuels produces carbon dioxide, which is a greenhouse gas.
  • The greenhouse gases in the atmosphere absorb infrared energy (heat). They emit some of the energy they absorb back towards earth, keeping it warm. This is called the greenhouse effect.
  • Most scientists agree that by increasing the amount of carbon dioxide in our atmosphere, we make the earth warmer.
  • This process is known as global warming.
  • In vehicles, engines dont burn all the fuel molecules. Some of them will come out as unburnt hydrocarbons.
  • Oxides of nitrogen (NO) are created when high temperature/pressure in a car engine cause nitrogen and oxygen to react together.
  • Hydrocarbons and nitrogen oxides react in the presence of sunlight to form ground level ozone, which is part of smog. It is a problem as it irritates peoples eyes, causes respiratory problems and can even cause lung damage.
  • Catalytic converters can be used to remove it.
  • Some fossil fuels contain sulfur, and when they are burnt sulfur dioxide gas is formed. This can cause acid rain.
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Chloroalkanes And CFCs.

A Free Radical- a particle with an unpaired electron.

  • Free radicals form when a covalent bond splits equally, giving one electron to each atom.
  • The unpaired electron makes them very reactive.

(http://www.scienceaid.co.uk/chemistry/organic/images/initiation.jpg)    The dot represents the unpaired electron.

  • Halogen reaction with alkanes in photochemical reactions (ones started by ultraviolet light).
  • A hydrogen atom is substituted (replaced) by chlorine or bromine. This is a free-radical substitution atom.

(http://chemwiki.ucdavis.edu/@api/deki/files/2090/=Slide4_(1).jpg?revision=1)

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Chloroalkanes And CFCs Part 2.

Stages of the reaction between chlorine and methane.

Step 1: Initiation Reaction.

  • Sunlight provides enough energy to break the Cl-Cl bond- this is photodissociation.
  • The bond splits equally and each atom gets to keep one electron. The atom becomes a highly reactive free radical, Cl`, because of its unpaired electron.

Step 2: Propagation Reaction. ( ` = unpaired electron).

  • Cl attacks a methane molecule: Cl`+ CH4 = CH3` + HCL.
  • The new methyl free radical, CH3`, can attack another Cl2 molecule:
  • CH3` + Cl2 = CH3Cl + Cl`.
  • The new Cl` can attack another CH4 molecule, until all the Cl2 and CH4 are used up.

Stage 3: Termination Reaction.

  • If two free radicals join together, they make a stable molecule. And the two unpaired electrons form a covalent bond.
  • If the chlorine is in excess dichloromethane, tri, and tetra can be formed. If the methane is excess the product will be chloromethane.
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Chloroalkanes And CFCs Part 3.

  • Chlorofluorocarbons (CFCs) are halogenalkane molecules where all of the hydrogen atoms have been replaced by chlorine and fluorine atoms. 

(http://ozonedepletioncausedbycfcs.weebly.com/uploads/2/8/2/1/28215559/7514040.png?205)      

  • Ozone in the upper atmosphere acts as a chemical sunscreen, It absorbs a lot of ultraviolet radiation from the sun, stopping it from reaching us. UV can cause sunburn or even skin cancer.
  • Ozone formed naturally when an oxygen molecule is broken down into two free radicals by UV radiation. The free radicals attack other oxygen molecules, forming ozone.
  • Chlorine free radicals are formed in the upper atmosphere when Cl-Cl bonds in CFCs are broken down by UV radiation.
  • These free radicals are catalysts. They react with ozone to form intermediate and O2.
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Chloroalkanes And CFCs Part 4.

  • CFCs are relatively unreactive, non-flammable and non-toxic. They used to be used as a coolant gas in fridges, as solvents, and as propellants in aerosols.
  • In the 1970s research by several different scientific groups demonstrated that CFCs were causing damage to the ozone layer. The advantage of CFCs couldnt outweigh the environmental problems they were causing, so they were banned.
  • Chemists have developed safer alternatives to CFCs which contain no chlorine such as HFCs and hydrocarbons.
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Halogenoalkanes.

A Halogenoalkane- is an alkane with at least one halogen attached. E.g. He, Ne, Ar, Kr, Xe, Rn.

  • Halogens are much more electronegative than carbon, so carbon-halogen bonds are polar.

  • The carbon is positive and the halogen is negative.
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Halogenoalkanes Part 2.

A Nucleophile- is an electron-pair donor. It donates an electron pair to somewhere that is electron defficient (Carbon).

  • OH- , CN- , and NH3 are all nucleophiles that react with halogenalkanes.
  • A nucleophile can react with a polar molecule by kicking out the functional group and replacing it.
  • This is called nucleophilic substitution reaction.

(http://revisionworld.com/sites/revisionworld.com/files/imce/mechanism%20nucleophilic.jpg)

  • The product of these reactions will always be the nucleophile.
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Halogenoalkane Reactions.

  • Halogenoalkanes can react with hydroxides to form alcohols.
  • For example, bromoethane can react to form ethanol. 
  • You have to use a warm aqueous sodium or potassium hydroxide, it is a nucleophilic substitution reaction.
  • Nitriles are formed by reacting halogenoalkanes with cyanide.
  • If you warm a halogenoalkane with ethanolic potassium cyanide, you get a nitrile. It is another nucleophilic substitution reaction.
  • Reacting halogenoalkanes with ammonia forms amines.
  • If you warm a halogenoalkane with excess ammonia, the ammonia swaps places with the halogen, and creates a further nucleophilic substitution.
  • The ammonium ion can react with the bromine ion to form ammonium bromide.
  • The amine group in the product still has a lone pair of electros. This means it is a nucleophile and may go on to further react itself.
  • The carbon-halogen bond strength decides its reactivity. For any reaction that occurs, this bond needs to break.
  • The C-F bond is the strongest, so fluoroalkanes undergo nucleophillic reactions slower.
  • The C-I has the lowest, so it breaks and substitutes more quickly.
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Halogenoalkane Reactions Part 2.

  • Halogenoalkanes can also undergo elimination reactions.
  • If you warm a halogenoalkane with hydroxide ions dissolved in ethanol instead of water, an elimination reaction happens and an alkene remains.
  • You have to first eat the mixture under reflux or you'll lose volatile stuff.(http://chemwiki.ucdavis.edu/@api/deki/files/3249/=image070.png?revision=1)
  • In an elimination reaction, a small group of atoms breaks away from a molecule. This group is not replaced by anything else.
  • The type of reaction carried out varies on its conditions. When halogenoalkanes are reacted with hydroxides, they undergo nucleophilic substitution or elimination.
  • You can influence which reaction will happen the most by changing the condition.
  • If aqueous conditions are used a nucleophilic substitution takes place.
  • If anhydrous conditions are used a elimination reaction takes place.
  • If you used a mixture of water and ethanol as solvent, both reactions happen.
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Alkenes Basics.

  • Alkenes have the general formula CnH2n.
  • They are just made of hydrogen and carbon, so are hydrocarbons.
  • They have atleast one C=C double covalent bond. Molecules with C=C bond are unsaturated because they can make more bonds with extra atoms in addition reactions.
  • Because there are two pairs of electrons in the C=C double bond, it has a really high electron density.
  • This makes alkenes pretty reactive.

(http://canadaconnects.ca/%3Cem%3Echemistry/photos/Ethylene2.png)

  • Bromine water can be used to test for alkenes.
  • If an alkene is shaken with orange bromine water, the solution quickly decolourises. 
  • Bromine is added across the C=C bond to form a colourless dibromoalkane, this happens by electrophilic addition.
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Alkene Reactions.

  • Alkenes can experience electrophilic reactions.
  • The double bond opens up and atoms are added to the carbon atoms.
  • Electrophilic addition reactions happen because the double bond has got a lot of electrons and is easily attacked by electrophiles.
  • Electrophiles are electron-pair acceptors they're usually a bit short of electrons, so are attracted to the electron rich areas.
  • E.g. positively charged ions, like H+ and NO2+.
  • E.g. polar molecules, like the delta + atom is attracted to places with lots of electrons.
  • Alkenes also undergo addition with hydrogen halides, to form halogenoalkanes.
  • Adding hydrogen halides to unsymmetrical alkenes forms two products.
  • If the hydrogen halides adds to an unsymmetrical alkene, there are two possible products.
  • The amount of each product formed depends on how stable the carbocation formed in the middle of the reaction is, this is known as carbocation intermediate.
  • Carbocations with more alkyl groups are more stable because the alkyl groups feed electrons toward the positive charge. The more stable carbocation is likely to form.
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Alkene Reactions Part 2.

  • Alkenes undergo electrophilic addition reactions with H2SO4.
  • Alkenes will react with cold concentrated sulfuric acid to form alkyl hydrogen sulfates. 
  • You can then convert the alkyl hydrogen sulfates formed into alcohols by adding water and warming the reaction mixture.
  • Just as with hydrogen halides, if you do this reaction with an unsymmetrical alkene, you get a mixture of products.
  • The one thats formed via the most stable carbocation intermediate will be the major product.

(http://www.chemguide.co.uk/mechanisms/eladd/proph2so4m.GIF)

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Addition Polymers.

  • Polymers- Long chain molecules formed when lots of small monomers join together.
  • Polymers can be nature, like DNA or synthetic, like polyethene.
  • Alkenes act as monomers and form polymers because their double bonds can open up and join together to make long chains. These polymers are called addition polymers.

(http://www.scienceaid.co.uk/chemistry/organic/images/polystyrene.png)

  • Polyalkene chains are saturated molecules. The main carbon chain of a polyalkene is also non-polar. These make addition polymers very unreactive.
  • Polyalkene chains are usually non-polar so the chains are held together by Van der Waals forces.
  • The longer the chains and the closer, the stronger the VDW are.
  • Long, straight chains = strong and ridgid. Polyalkanes are short, unbranched.
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Addition Polymers Part 2.

  • Adding a plasticiser to a polymer makes it more flexible.
  • The plasticiser molecules get between the polymer chain and pushes them apart.
  • This reduces the strength of the intermolecular forces between the chains, so they can slide around more, making them easier to bend.
  • Polymers are made up of repeating units. The repeating units of addition polymers look very similar to the monomer, but with a double bond opened out.

(http://shawonnotes.ucoz.com/IGCSE_Chemistry/images/clip_image110.jpg)

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Alcohols.

  • The functional group in the alcohol homologous series is the hydroxyl group (-OH).
  • An alcohol is primary, secondary, tertiary, this depends on which carbon atom the -OH group is bonded to.

(http://chemwiki.ucdavis.edu/@api/deki/files/4412/=image087.png?revision=1)

  • Alkenes can be made by eliminating water from alcohols in a dehydration reaction.
  • This reaction allows alkenes to be produced from renewable resources.
  • This is important because it means polymers can be produced, without needing oil.
  • One of the main industrial uses for alkenes is as a starting material for poylmers.
  • When dehydrating ethene into ethanol, ethanol is heated with a concentrated sulfuric acid catalyst.
  • The product is usually in a mixture with water, acid and reactant, so the alkene has to be seperated out.
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Distillation Of Alcohol.

  • The products of organic reactions are often inpure- so you've got to know how to purify them.
  • In the dehydration of alcohols to form alkenes, the mixture at the end contains the product, the reactant, acid and water. 
  • To get pure alkene, you need a way to seperate it from the other substances.
  • Distillation- a technique used seperate mixtures, at their different boiling points.
  • Often further seperation and purification is needed.

Producing cyclohexene from cyclohexanol.

  • Step 1 = Distillation.
  • Step 2 = Seperation.
  • Step 3 = Purification.
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Ethanol Production.

  • The industrial method for producing alcohols is to hydrate an alkene using steam in the presence of an acid catalyst.

(http://www.chemguide.co.uk/mechanisms/elim/dhethanolms.GIF)

  • Ethanol can be produced by the hydration of ethene by steam at 300 celsius and a pressure of 60 atm. 
  • It also needs a solid phosphoric (V) catalyst.
  • In current time, hydration of ethene is a widely used technique in the industrial production of ethanol. The ethene comes from cracking heavy fractions of crude oil. However, in the future when crude oil begins to become sparse, ethene will become very expensive. Therefore, producing ethanol by fermentation, using renewable materials will become important.
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Ethanol Production Part 2.

  • Ethanol can be produced by the fermentation of glucose.
  • Fermentation is an exothermic process, carried out by yeast in anaerobic conditions.
  • The yeast produces an enzyme which converts sugars into ethanol and carbon dioxide.
  • The enzyme works at an optimum temperature of around 30-40 degrees celsius.
  • Once formed, ethanol is seperated from the rest of the mixture, using fractional distillation.
  • Fermentation in low-tech and uses cheap equipment, aswell as renewable resources.

Ethanol is increasingly being used as a few, and when made using fermentation is called a biofuel.

Biofuel- a fuel that is made from biological material that has recently died. These types of fuels have some advantages over normal fuels.

  • They are renewable, maiking them more sustainable.
  • They are carbon neutral.
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Oxidation Of Alcohols.

  • The simplest way to oxidise alcohols is to burn them. However, this does not give off interesting products.
  • The oxidising agent acidified potassium dichromate (VI), K2Cr2O7 for mild oxidation. The orange dichromate ion Cr2O72- is reduced to the green chromium (III) ion.
  • Primary alcohols are oxidised to aldehydes and then to carboxylic acids.
  • Secondary alcohols are oxidised to ketones only.
  • Tertiary alcohols aren't oxidised.
  • Aldehydes and Ketones are carbonyl compounds- they have the functional group C=O.
  • Their general formula is CnH2nO.
  • Carboxylic acids have the functional group COOH and have the general formula CnH2n+1COOH.
  • Aldehydes- have a hydrogen and one alkyl group attached to the carbonyl atom.
  • Aldehydes- have the suffix -al. The fuctional group is always on carbon 1.
  • Ketones- have two alkyl groups attached to the carbonyl carbon atom.
  • Ketones- have the suffix -one. When five or more carbons, say function group one.
  • Carboxylic Acid- have a COOH group at the end of their carbon chain.
  • Carboxylic Acid- have the suffix -oic acid.
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Oxidation Of Alcohols Part 2.

  • Primary alcohols will oxidise to aldehydes and carboxylic acids, how fat an alcohol is oxidised can be controlled, specifically the reaction conditions.
  • This can be done by gently heating the ethanol with potassium dichromate (VI) and sulfuric acid in a test tube, this produces ethanal (an aldehyde). 
  • However, controlling the heat is difficult, and the aldehyde is often oxidised to form ethanoic acid.
  • For just the aldehyde to remain, it needs to be removed from the oxidising solution as soon as it forms. 
  • This is done using a distillation apparatus, so the aldehyde is distilled off immediately.
  • To produce the carboxylic acid, the alcohol must be vigorously oxidised and mixed with an excess oxidising agent and heated under a reflux.
  • Heating it under reflux means that the temperature can be increased, without losing volatile solvents, reactants or products.
  • Any vapourised compounds will cool, condense and drip back into the mixture, this creates the oxidation.
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Oxidation Summary.

  • Refluxing a secondary alcohol e.g. propan-2-ol, with acidified dichromate (VI) will produce a ketone.
  • Ketones are not oxidised easily, so even prolonged refluxing will not produce much more.
  • Tertiary alcohols do not react with potassium dichromate (VI) at all, the solution will stay orange.
  • The only way to oxidise tertiary alcohols is by burning them.
  • Aldehydes and Ketones can be distinguished using an oxidising agent, aldehydes are easily oxidised, but ketones aren't.
  • Fehlings solution and benedicts solution are both deep blue Cu2+ complexes, which reduces to brick-red Cu2O, when warmed with an aldehyde, but stay blue with a ketone.
  • Tollens reagent is [Ag(NH3)2]+  is reduced to silver when warmed with an aldehyde, but not with a ketone. The silver would coat the inside of the apparatus to form a silver mirror.
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Tests For Alcohols.

Alcohols can be oxidised using acidified potassium dichromate (VI), however it can also be tested whether you have a primary, secondary or tertiary alcohol:

  • Add 10 drops of alcohol, to 2 cm3 of acidified potassium dichromate solution in a test tube.
  • Warm the mixture gently in a hot water bath.
  • The colours will then change:
  • Primary- the orange solution slowly turns green as an aldehyde forms.
  • Secondary- the orange solution slowly turns green as a ketone forms.
  • Tertiary- nothing happens.

The error with this test is that it shows the same results for primary and secondary alcohols. To find out which you began with you will have to repeat the experiment, and collect some of the products. Heres how:

  • Add excess alcohol to 2 cm3 of acidified potassium dichromate solution in a round bottomed flask.
  • Set up the flask as part of distillation apparatus.
  • Gently heat, until it is oxidised.
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Testing For Aldehyde's And Ketone's.

There are three main solutions to distinguish between aldehydes and ketones. These are: Fehling's solution, Benedict's solution and Tollens' reagent.

Fehling's and Benedict's solutions work in exactly the same way:

  • Add 2 cm3 of Fehling's or Benedict's solution to a test tube. They both appear as a clear blue solution.
  • Add 5 drops of the aldehyde or ketone to the test tube.
  • Put the test tube in a hot water bath to warm it for a few minutes.
  • Aldehyde- the blue solution will give a brick red precipitate.
  • Ketone- nothing happens.
  • However, using Tollens' reagent is slightly different, as the reagent must be made.
  • Put 2 cm3 of 0.10 mol dm-3 silver nitrate solution in a test tube.
  • Add a few drops of dilute sodium hydroxide solution, a light brown precipitate forms.
  • Add drops of dilute ammonia solution until the brown precipitate dissolves.
  • Place the test tube in a hot water bath and add 10 drops of aldehyde or ketone.
  • Aldehyde- a silver mirror forms on the walls of the test tube.
  • Ketones- nothing happens.
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Testing For Other Functional Groups.

Testing for an alkene (the presence of double bonds):

  • Add 2 cm3 of the solution that you want to test to a tube.
  • Add 2 cm3 of bromine water to the test tube.
  • Shake the test tube.
  • Alkene- the solution will decolourise (from orange to colourless)
  • Not Alkene- nothing happens.

Another thing to test for can be carboxylic acids. These carboxylic acids react with carbonates to form a salt, carbon dioxide and water. This can be used to test whether a solution is a carboxylic acid:

  • Add 2 cm3 of the solution that you want to use to a test tube.
  • Add 1 small spatula of solid sodium carbonate.
  • If the solution begins to fizz, bubble the gas that it produces through some limewater in a second test tube.
  • Carboxylic acid- the solution will fizz. The carbon dioxide gas that is produced will turn limewater cloudy.
  • Not carboxylic acid- nothing happens.
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Analytical Techniques- Mass Spec.

  • Mass spec can be used to find the relative molecular mass (Mr) of a compound.
  • In the mass spec, a molecular ion is formed when a molecule loses an electron.
  • The molecular ion produces a molecular ion peak on the mass spec of a compound.
  • For any compound, the mass/charge value of the molecular ion peak will be the same as the molecular mass of the compound.

Question: The mass spectrum of a straight chain alkane contained a molecular ion peak with m/z 72. Identify the compound.

  • The m/z value of the molecular ion peak is 72, so the Mr of the compound must be 72.
  • If you calculate the molecular masses of the first few straight-chain alkanes, you will find that the one with a molecular mass of 72 is pentane.
  • Mr of pentane- (5 x 12) + (12 x 1) = 72.
  • Some mass specs can measure the atomic and molecular masses extremely accurate. These are known as high resolution mass specs.
  • These are useful for identifying compounds that have the same Mr, when rounded.
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Analytical Techniques- Infrared Spec.

  • Infrared spec, a beam of IR radiation is passed through a sample of a chemical.
  • The IR radiation is absorbed by the covalent bonds in molecules, increasing their vibrational energy.
  • Bonds between different atoms absorb different frequencies of IR radiation. Bonds in different places in a molecule absorb different frequencies too. The stats below show frequences and absorbs.
  • N-H (amines) e.g. methylamine, wavenumber of 3300-3500.
  • O-H (alcohols) e.g. ethanol, wavenumber of 3230-3550.
  • C-H e.g. most organic molecules, wavenumber of 2850-3300.
  • O-H (acids) e.g. carboxylic acid, wavenumber of 2500-3000.
  • C-N (triple bond) e.g. nitriles such as ethanenitrile, wavenumber of 2220-2260.
  • C=O e.g. aldehydes and ketones, wavenumber of 1680-1750.
  • C=C e.g. alkenes, wavenumber of 1620-1680.
  • C-O e.g. alcohols and carboxylic acids, wavenumber of 1000-1300.
  • C-C e.g. most organic molecules, wavenumer of 750- 1100.

An infrared spec produces a graph that shows what frequences of radiation the molecules are absorbing.

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Analytical Techniques- Infrared Spec Continued.

  • The region between 500 cm-1 and 1500 cm-1 is called the fingerprint region. It is unique to a particular compound. A computer database is used to check this region of an unknown compounds IR spectrum against those of known compounds.
  • If it matches one of them, you know what the molecule is.
  • Infrared spec can also be used to find out how pure a compound is, and identify any impurities.
  • Impurities produce extra peaks in the fingerprint regions.
  • Some of the electromagnetic radiation emitted by the sun reaches the earth and is absorbed.
  • The earth then re-emits some of it as infrared radiation (heat).
  • Molecules of greenhouse gas, e.g. carbon dioxide. In the atmosphere absorb this infrared radiation. They then re-emit some of it back towards the earth, keeping us warm. This is called the greenhouse effect.
  • Human activities have caused a rise in greenhouse gas concentrations.
  • This means more heat is being trapped and the earth is getting warmer- this is global warming.
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