CHEM UNIT 4: FURTHER ORGANIC CHEMISTRY

  • ISOMERISM
  • ALDEHYDES AND KETONES
  • ORE ON ALDEHYDES AND KETONES
  • CARBOXYLIC ACIDS
  • ESTERS
  • ACYL CHLORIDES
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  • Created by: Chynna
  • Created on: 19-03-13 12:53

ISOMERISM

Structural isomers have different structural arrangements of atoms - same molecular formula

stereoisomers are arranged differently in space - same molecular formula and arranged in the same way - difference is te orientaion of the bonds in space - E/Z Isomerism and optical isomerism

E/Z Isomerism happens because there's no rotation about the double bond

only happens when

  • there's a C=C bond, like in alkenes - groups attached to the carbons are fixed in position
  • 2 different groups are attached to each of the double bonded carbon atoms
  • sometimes called geometric isomerism 'cis' - Z-isomer and 'trans' - E-isomer
  • if the 2 carbon atoms have their 'higher priority group' on opposite sides - E isomer
  • if the 2 carbon atoms have their 'higher priority group' on same side - Z isomer
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ISOMERISM

Optical isomers are mirror images of each other

  • another type of stereoisomerism
  • chiral carbon - carbon atom that has 4 different groups attached to it - possible to arrange the groups around the atom in 2 different ways so that 2 different molecules are made = enantiomers or optical isomers
  • enantiomers are mirror images of each other and cannot be superimposed
  • optical isomers are optically active - rotae plane-olarised light, one rotates in a clockwise direction, the other anticlockwise

racemate - mixture of both optical isomers

racemate (racemic mixture) - contains equal quantities of each enantiomer of an optically active compound

  • don't show any optical activity - the enantiomers cancel each other's light rotating effect
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ISOMERISM

You can use optical activity to work out a reaction mechanism

e.g. Nucleophilic substitution

SN1 

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ALDEHYDES AND KETONES

-Contain a carbonyl group - C=O

Aldehydes have their C=O at the end of the carbon chain (-al)

Ketones have their carbonyl group in the middles (-one)

-don't hydrogen bond with themselves

  • don't have polar O-H bond so cannot hydrogen bond with themselves
  • this means they have lower boiling points than their equivalent alcohols (which can form H bonds)

-but they can hydrogen bond with water - have a polar C=O bond

  • small aldehydes and ketones will then dissolve in water - if the carbon chain is longer the intermolecular forces between the long chains are stronger than the H bonds that could form and the compound wonm't dissolve.
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ALDEHYDES AND KETONES

Hydrogen cyanide will react with carbonyls by nucleophilic addition

  • produce hydroxynitriles (molecules with a CN and OH group)
  • nucleophilic addition - nucleophile attacks the molecule and an extra group is added to it

Hydrogen cyanide is a highly toxic gas so a solution of acidified potassium cyanide is used instead in a lab experiment to reduce the risk and it is still done in a fume cupboard.

evidence for the reaction mechanism - when you react it with an aldehyde or an assymetric ketone you get a racemic mixture - carbonyl group is attcked from both sides of the double bond = equal amounts of the 2 products

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ALDEHYDES AND KETONES

Brady's reagent (2,4 - DNPH) tests for a carbonyl group - forms a bright orange precipitate if a carbonyl group is present - only tests for aldehydes and ketones!! (not carboxylic acids!!)

melting point of the precipitate identifies the carbonyl compound - orange precipitate is a derivative of the carbonyl compound - purified by recrystallisation which has a different melting point from other derivatives of other carbonyl compounds - compare it to a table of known melting points, you can identify the compound

testing for aldehydes

  • all work on the idea the aldehydes are easily oxidised to a carboxylic acid and another compound is reduced - so a reagent canges colour as it is reduced
  • Tollen's reagent - colourless solution of silver nitrate dissolved in aqueous ammonia, if heated in a test tube with an aldehyde, a silver mirror forms
  • Fehling's solution - blue soltuion of complexed copper (II) ions dissolved in sodium hydroxide - brick red precipitate of copper (I) oxide produced with an aldehyde
  • benedict's solution - same as fehling's but copper (II) oxide ions are dissolved in sodium carbonate instead.
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ALDEHYDES AND KETONES

Aldehydes oxidise to carboxylic acids and ketones do not

  • heat an aldehyde with acidified dichromate (VI) ions = carboxylic acid - the dichromate (VI) ions are the oxidising agent [O]. Potassium dichromate (VI) with dilute sulfuric acid is usually used.
  • ketones do not because the dichromate (VI) ions are not a strong enough oxidising agent

you can reduce aldehydes and ketones back to alcohols

  • usually use LiAlH4 in dry diethyl ether - very powerful reducing agent, which reacts violently with water, bursting into flames

some carbonyls will react with iodine

carbonyls that contain a methyl carbonyl group react when heated with iodine in presence of an alkali - if there is a methyl carbonyl group = yellow precipitate of triiodomethane (CHI3) and an antiseptic smell

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CARBOXYLIC ACIDS

  • Contain COOH
  • -oic acid
  • weak acids - in water they'll partially dissociate into carboxylate ions and H+ ions

are very soluble

  • are polar molecules - electrons are drawn towards the O atoms - have relatively high boiling points
  • polar bonds make small carboxylic acids very soluble in water - frm H bonds with water molecules - H and O atoms on different molecules are attracted to each other
  • solubility decreases as the length of the chain increases because longer chains have stronger intermolecular forces between them - which could be stronger than the H bonds and so it won't dissolve
  • in pure, liquid carboxylic acids, dimers can form - when a molecule H bonds with just one other molecule which increases its size which increases the intermolecular forces and hence also the boiling point.
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CARBOXYLIC ACIDS

can be formed from alcohols, aldehydes and nitriles

  • oxidation of primary alcohols and aldehydes
  • hydrolysis of nitriles -  reflux the nitrile with w/ dilute hydrochloric acid and then distil off the carboxylic acid

carboxylic acids react with alkalis and carbonates to form salts

  • carboxylic acids are neutralised by aqueous alkalis to form salts and water
  • react with carbonates to form CO3(2-) or hydrogencarbonates (HCO3-) to form a salt, CO2 and water
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CARBOXYLIC ACIDS

Other reactions you need to know:

  • reduction of a carboxylic acid - use LiAlH4 in dry diethyl ether - reduces it right down to an alcohol in one go.
  • mix it with phosphorus (V) chloride = acyl chloride
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CARBOXYLIC ACIDS

carboxylic acid conc can be estimated using titration

citric acid is found in fruit juices. use reaction between carboxylic acids and alkalis to find out how much citric acid there is in fruit juice

  • measured amount of fruit juice is placed in a flask and phenolphthalein (indicator) added - turns pink in alkaline conditions.
  • sodium hydroxide solution with a known conc. is added slowly from a burette
  • the -COOH groups are neutralised by the NaOH - end point is when indicator has turned pink for at least 30 seconds. citric acid is a very weak acid so reaction is slow
  • once you know how much NaOH is needed to neutralise the carboxylic acid - you cna work out the conc. of the citric acid in the fruit juice.
  • e.g. 12.5ml of 0.1mol/dm3 NaOH neutralises exactly 25ml of orange juice. What is the conc. of citric acid in the juice?
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CARBOXYLIC ACIDS

alcohols react with carboxylic acids to form esters

  • heat a carboxylic acid w/ an alcohol in presenc of an acid catalyst = ester
  • e.g. reflux ethanoic acid with ethanol and conc. sulfuric acid = ethy ethanoate
  • reaction is reversible - need to seperate out the products by distillation as it is formed, collecting the liquid that comes off just below 80'c
  • product is then mixed w/ sodium carbonate solution to react with any acid that might have snuck in. the ethyl ethanoate forms a layer on top of the aqueous layer and can be easily separated using a separating funnel.
  • ethyl ethanoate is often used as a solvent in chromatography and as pineapple flavouring  
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ESTERS

  • -COO-
  • made by reacting an alcohol w/ a carboxylic acid 

esters are hydrolysed to form alcohols

  • acid hydrolysis - splits the ester into an acid and an alcohol - reverse of condensation, reflux ester w/ a dilute acid (e.g. hydrochloric / sulfuric)
  • bade hydrolysis - reflux ester w/ dilute alkali such as sodium hydroxide (NaOH) = carboxylate ion and an alcohol
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ESTERS

Base hydrolysis of esters is used to make soaps

  • veg oils and animal fats are actually triesters (or triglycerides)
  • made from esterification of glycerol with fatty acids. each OH group on the glycerol is replaced with a fatty acid joined by an ester bond
  • as with other esters you can hydrolyse fats and oils by heating them with NaOH. the sodium salt you get is a soap
  • so to make a soap: heat fat oil w/ conc. solution of NaOH + allow it to cool then add some saturated NaCl solution and soap will separate out as a crust on the surface of the liquid.
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ESTERS

Esters react with alcohols to make new esters - process called transesterification - allows you to swap a different alcohol for the alcohol part of an ester

making low fat spreads to replace butter

  • saturated fats have no double bonds in their fatty acid chains
  • have higher melting temps - molecules are straighter which lets them pack closer together - giving greater intermolecular forces
  • low fat spreads used to be made by hydrogenation - unsaturated veg oil is reacted with hydrogen to remove its double bonds - produces a solid fat that spreads easily
  • but this also produces trans isomers of the fatty acids which are linked w/ various diseases
  • manufacturers now use transesterification to convert veg oils to diff esters w/ higher melting temps - avoids the hydrogenation process

mqaking biodiesel from veg oils

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ESTERS

reactions between dicarboxylic acids and diols make polyesters

carboxyl groups react w/ hydroxyl groups to form ester links

  • Polyester fibres are strong, flexible qnd abrasion-resistant
  • you can treat polyesters (by stretching and heat-treating them) to make them stronger
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ACYL CHLORIDES

  • -COCl
  •  
  • -oyl chloride

acyl chlorides can easily lose their chlorine

  • react with cold water to produce a carboxylic acid
  • vigorous reaction with an alcohol at room temp to make an ester
  • violent reaction at room temp w/ ammonia to make an amide
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ACYL CHLORIDES

  • violent reaction at room temp with an amine = N substituted amide

 

 

 

each time Cl is substituted by an oxygen or nitrogen group and HCl fumes are given off

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Comments

PrimeEpoch

For this bullet point, it might be useful to indicate that it only has an effect on the plane of polarised light. It's like the difference between rotating a line and rotating a circle for an analogy. 

  • don't show any optical activity - the enantiomers cancel each other's [polarised] light rotating effect

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