Chemistry

Chemistry A2 - module 4 - Further Organic Chemistry

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Isomerism

Isomerism - Isomers with the same molecular formulae but atoms arranged different.

Structural - Same formula, but a different structure.

Positional - Different position of the functional group.

Chain - The arrangement of the carbon atoms are different.

Functional Group - Same molecular formula but a different functional group.

Sterioisomerism- same molecular and structural formula, bonds orientated differently in space.

Geometrical- The displayed formula is different, cis-trans isomerism.

Optical Isomerism- contain an asymmetric carbon, so have no plane of symmetry. Isomers are non- super imposable on one another.

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Optical Isomerism

When four different atoms are joined to a carbon they are said to be assymetric, as it has no centre, plane, or axis of symmetry. The moelcule is described as chiral, centre carbon = chiral carbon.

Optical isomers are known as enantiomers.

They cannot be super imposed on one another.

Same chemical and physical properties, rotate polarized light either clockwise or anti-clockwise.

The dextroenantiomer (+) rotates it clockwise. The Laevoenantiomer (-) rotates it anti-clockwise.

If equal amounts of + and - are present then the mixture appears optically inactive and is called a racemic mixture of racemate.

Chemical reactions tend to produce racemate while biological reactions produce single enantiomers.

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Aldehydes and Ketones

Oxidation- aldehydes are readily oxidised to carboxylic acids.

Tollen's reagent with aldehydes - colourless - gently warm - silver mirror

Fehling's solution with aldehydes - blue - gently warm - brick red precipitate.

Ketones show a negative result with both tollen's reagent and fehling's solution.

Reduction of Aldehydes and ketones

NaBH4 - addition of hydride ion, reduced to alcohols by sodium tetrahydridoborate(III) in methanol or LiAlH4 in ethanol.

Aldehyde - RCHO + 2[H] = RCh2OH primary alcohol

Ketone - RCOR' + 2[H]= RCH(OH)R' secondary alcohol

NaBH4 = [H]. Does not reduce alkenes so compounds with C=C or C=O. Not free in solution

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Reduction of Aldehydes and Ketones 2

Catalytic hydrogenation - addition of hydrogen using a catalyst

A nickel or platinum catalystwill add hydrogen across the C=O bond, saturating all double bonds in aldehydes and ketones to produce alcohols.

RCHO + H2 = RCH2OH

RCOR + H2 = RCH(OH)R

Addition of Hydrogen Cyanide

Aldehydes and ketones react with HCN to produce hydroxynitriles. HCN is often formed in the reaction.

e.g. Ch3CHO + HCN = CH3CH(OH)CN

ethanal + hydrogen cyanide = 2-hydroxypropanenitrile

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Carboxylic acids

Carboxylic acids have two groups, the carbonyl group (C=O) and the hydroxyl group(-OH), so the -COOH group is known as the carboxyl group.

Production - Oxidation of a primary alcohol or an aldehyde. The aldehyde can thenbe further oxidised by refluxing with excess oxidising agent.

Oxidising agent = acidified potassium dichromate(VI)

Acid Reactions

Carboxylic acids are weak acids but they will release carbon dioxide on reaction with carbonates or hydrogencarbonates. This can be used as a test for carboxylic acids.

2 propanoic acid + sodium carbonate = sodium propanoate + carbon dioxide + water

Carboxylic acids are weak acids so will neutralise bases like NaOH.

Ethanoic acid + sodium hydroxide = sodium ethanoate + water

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Esterification

Carboxylic acids react with alcohols (ROH) in the presence of a strong acid catalyst (conc sulphuric acid)

Acid + Alcohol = ester + water

Esterification can be thought as a condensation reaction since it involves the elimination of water. More accurately, it is an addition-elimination reaction. It is teh oxygen from the alcohol that forms the 'bridging' oxygen, not the oxygen on the acid. If the alcohol's oxygen contains the O18 isotope, it can be traced to the ester produced in the reaction. This is isotopic labelling.

The carboxylate ion

When the acid loses a proton and becomes dissociated, we often write the structure as having a minus charge on the oxygen which was attached to the hydrogen.

The point charge is stabilised by being spread evenly onto the two oxygen atoms.

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Esters

The alcohol used determines the start of the esters name and the acid determines the ending of the ester. Esters have strong sweet smell and are naturally found in foods such as fruits. They are used as artificial flavourings. Esters are almost insoluble in water due to a lack of an OH group in their structure, but their molecules are slightly polar, due to carbon-oxygen bonds. They make good solvents for polar organic compounds e.g. glue. Esters are added to plastics to make them softer and more flexible e.g. in car interior, upholstery and children's toys. They allow the chains of polymers to slide past each other, increasing flexibility.

Esters undergo hydrolysis (in alkaline conditions) Alkaline hydrolysis using NaOH produces an alcohol and the sodium salt of teh carboxylic acid.

Methyl Propanoate + Sodium Hydroxide = Sodium Propanoate + methanol

Alkaline hydrolysis is known as saponification, when excess dilute sulphuric or hydrochloric acid is added to the sodium salt of a carboxylic acid then the carboxylic acid forms.

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Triesters or triglycerides

Natural fats and oils are triesters. They are esters of long chained carboxylic acids and propane-1.2.3-triol.

Fats contain mainly saturated fatty acids and oils contain fatty acids with some degree of saturation.

Since glycerol contains three alcohol groups, the triesters it forms have three carboxylic acids attached. The 'R' groups of the triester may be teh same or different but are often straight chained.

Formation of soap

Triesters can be hydrolysed by boiling with sodium hydroxide solution.

They form glycerol and the sodium salts of the long-chain carboxylic acids.

These sodium salts are used in the manufacture of soap.

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Acylation

Acyl chlorides and acid anhydrides are derived from carboxylic acids.

The mechanism involves nucleophilic addition followed by elimination - nucleophilic addition-elimination.

Aspirin

Aspirin is an analgesic (painkiller) and an anti-pyretic (a reducer of body temperature).

It is made by acylating 2-hydroxybenzoic acid or salicyclic acid.

Ethanoic anhydride is used as the acylating agent in preference to ethanoyl chloride as it is;

1. cheaper, 2. less corrosive, 3. less ssusceptible to hydrolysis, 4. safer

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Acylation 2

Acyl chloride + water = carboxylic acid + hydrogen chloride

Acid anhydride + water = carboxylic acid

Acyl Chloride + alcohol = ester + hydrogen chloride

Acid Anhydride + alcohol = ester + carboxylic acid

Acyl Chloride + Ammonia = Acid Amine + Hydrogen Chloride

Acid Anhydride + Ammonia = Acid Amide + Carboxylic Acid

Acyl Chloride + Primary Amine = N-substituted acid amide + Hydrogen Chloride

Acid Anhydride + Primary Amine = N-substituted acid amide + Carboxylic acid

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Nitration of Benzene

Conditions: concentrated nitric and sulphuric acid, warm

Generating the electrophile: conc sulphuric acid acts as a catalyst which, when added to nitric acid, produces the nitronium ion.

H2SO4 + HNO3 = HSO4- + H2NO3+

The protonated nitric acid breaks down forming the nitronium ion NO2+

H2NO3+ = H2O + NO2+

C6H6 + HNO3 = C6H5NO2 + H2O

TNT an explosive, is formed from methylbenzene by nitration.

The nitro- group in benzene derivatives can be reduced by using tin and conc hydrochloric acid to produce aromatic amines. These are used to make synthetic dyes.

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Friedel-Crafts reactions - Electrophilic substitut

Alkylation - When chloroethane, benzene and aluminium chloride are warmed, ethylbenzene if formed (under anhydrous conditions)

C6H6 + CH3Ch2Cl = C6H5CH2CH3 + HCl

Regeneration of AlCl3 catalyst - H+ + [AlCl4]- = AlCl3 + HCl

Acylation- Ethanoyl Chloride, benzene and aluminium chloride with an aluminium chloride catalyst.

C6H6 + CH3COCl = C6H5COCH3 + HCl

The catalyst AlCl3 is regenerated.

The reaction can be stopped when one RCO+ has reacted, as this group withdraws electrons from the ring, reducing the chance of electrophilic attack.

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Amines

Preparation-They are produced from haloalkanes and ammonia by nucleophilic substitution. They are produced by reduction of nitriles with nickel catalysts. Aryl amines can be prepared by reduction of nitro derivatives using a mixture of tin or iron and concentrated hydrochloric acid.

Properties- The nitrogen atom in the amine molecule can donate its lone pair of electrons to other species. This means that amines behave as bases, nucleophiles and ligands. As bases - they donate a lone pair to a proton. As nucleophiles they donate a lone pair to a slightly positive carbon atom.

Nitrogen's lone pair can form a coordinate bond with a proton.

As they are proton acceptors, primary amines are Bronsted-Lowry bases.

In water they produce -OH ions so are weak alkalis.

The basicity of the amino group depends on the availability of the lone pair. Primary aliphatic amines < Ammonia < Aromatic Aryl Amines

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Nucleophilic Properties of Amines

1. With Haloalkanes (and ammonia)

Propylamine is produced from bromoethane, to produce a primary amine. The product further reacts with bromoethane in successive substitution reactions to form dipropylamine, a secondary amine. The product of this reaction can further react with bromoethane to form tripropylamine, a tertiary amine and a quaternary ammonium salt.

No more protons can be removed from the nitrogen and the positive charge resides on the nitrogen atom, so the quaternary ammonium ion cannot continue to behave as a nucleophile.

2. With acyl chlorides and acid anhydrides

Primary amines react with acyl chloride and acid anhydrides to form substituted amides.

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Amino Acids

Their general formula is RC*H(NH2)COOH. They exhibit optical isomerism if they have a carbon atom attached to four different groups. They have both acidic and basic properties, they are amphoteric.

As acids: the acidic carboxyl group (-COOH) donates a proton, here reacting with a base to form a salt plus water.

As bases: the basic amine group (-NH2) accepted a proton from an acid to form a salt.

Pure amino acids are white crystalline solids which easily dissolve in water and have high melting points. Zwitterions in pure state.

At high pH the proton on the -NH3+ is removed leaving an overall negative charge. At Low pH the -COO- group accepts a proton giving an overall positive charge.

At intermediate pH the negative and positive charges are equal and the amino acid will have no charge. The positive and negative charges in the zwitterion cause ionic bonds which are stronger than the expected hydrogen bonding, so ave high melting points.

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Proteins

Natural sequences of amino acids joined together by an amide bond called a peptide link. Join to form a polymer called a polypetide. They join up be addition-elimination reactions which involve the loss of water. They are either fibrous or glomerous.

Hydrolysis of proteins-water is added which breaks the peptide bond, this produces individual amino acids. The protein is refluxed with concentrated hydrochloric acid. If this is done form one end of the polymer, one amino acid at a time, it reveals the amino acids present and can give clue to the primary structure of the protein.

Hydrogen bonding in proteins- determines the secondary structure of the polymer chains. They usually produce an alpha helix or a beta pleated sheet.

The presence of many hydrogen bonds makes the protein very stable.

Hydrogen bonding, ionic bonding and disulphide bridges are important in forming the secondary structure.

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Addition polymers

These form directly from compounds containing C=C. They are referred to as chain-growth polymers as they form by the addition of a monomer to the end of the chain usually by free radical mechanism. Reaction conditions involve a high prssure and an initiator, which provides the free radicals needed to start the reaction. The length of the polymer chain can vary depending on the properties required.

Polyethene - shopping bags, cling film. Poly(styrene) - protection in packaging, thermal insulation. PVC - fake leather, pipes, flooring. Poly(propene) - washing up bowls, ropes. PTFE - coat electric irons and frying pans for non-stick properties. Perspex - Fake/safety glass.

Good Points - Used to make many products that do not affec the substances they are in contact with, like food or chemicals.

Bad Points - Non-biodegradable so difficult to dispose of. Some are flammable. Some give off toxic fumes on incineration.

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Condensation Polymers

When two monomers which functional groups at both ends join, and water is eliminated.

Polyesters - If a dicarboxylic acid and a diol react long chain polyesters form

RCOOH + R'OH = RCOOOR' + H2O

Polyamides - When a dicarboxylic acid and a diamine react a chain reaction occurs and a long chain condensation polyamide or amino acid forms.

RCOOH + R'NH2 = RCONHR' + H2O

Nylon-6,6 is formed from the condensation reaction between hexanedioic acid and 1,6-diaminohexane.

Hydrolysis of Polyesters and Amides- Both polyesters and polyamids undergo hydrolysis in acid or alkaline conditions so are biodegradable.

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Mass Spectrometry

A molecule is vaporised and ionised to form a molecular ion. Which produces a molecular ion peak (last but one).

Fragmentation is when a covalent bond in the molecular ion breaks due to electron impact and produces an ion and a free radical.

Stable ions like the carbocations CH3+ and CH3CH2+ and acyliumions CH3CO+ and CH3CH2CO+ are common and able to hold a positive charge.

Identifying fragments helps to determine the molecular structure and identify between isomers.

15 - CH3+ 29 - CH3CH2+ 31 - CH2OH+ 43 - (CH3)2CH+ or CH3CO+

57 - Ch3Ch2CO+ 77 - C6H5+ 91 - C6H5CH2+

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Infra-red Spectroscopy

Infra-red spectroscopy determines the functional groups present and the degree of purity. Certain bonds have a natural tendency to vibrate and do so at certain frequencies. If radiation across the full frequency range is passed through a substance, the bonds absorb energy corresponding to their natural frequency of vibration.

Fingerprint region - Each compound has its own individual fingerprint region which can be used to identify compounds by comparison with the spectra of known compounds. 1500-400 cm-1

The intensity of absorbancies varies depending on the polarity of the bonds.

O-H gives a very broad absorption peak

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Nuclear Magnetic Resonance (NMR)

Chemical Shift is a measure of how much a proton is shielded and is measured in parts per million (ppm) compared to a standard or reference compound TMS. TMS is used as its twelve methyl protons are well shielded so resonate up field. They are equivalent so produce one large peak away from most others, can be easily extracted.

Number of Absorption Peaks - this tells you the number of non-equivalent protons.

Position of Absorption Peaks - tells you what teh proton may be hear to.

Intensities of the absorption - tells you the relative heights of the peak and so tells you the relative numbers of protons in a certain environment generating that peak.

The Splitting - tells you about the neighbouring protons.

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