A2-Level Organic

Acid Anhydrides

  • Acid anhydrides are made from two carboxylic acids joined together in a condensation reaction.






     

  • Acid anhydrides react in similar ways to acyl chloride but are less reactive, making them generally safer to use. 

  • Acid Anhydride + Alcohol → Ester + Carboxylic Acid
     

Acyl Chlorides

  • Acyl chlorides are highly reactive and easily react with nucleophiles, they have the functional group:






     

  • They are prepared by reacting a carboxylic acid with phosphorus pentachloride (PCl  ) in anhydrous conditions (acyl chlorides react easily with water). 

  • Acyl chlorides react with nucleophiles bonded to a hydrogen atom in addition-elimination reactions:







    Key reactions 
     

  • Acyl Chloride + Water → Carboxylic Acid (+ HCl)

  • Acyl Chloride + Alcohol → Ester (+ HCl)

  • Acyl Chloride + Ammonia → Primary Amide (+ Ammonium Chloride)

  • Acyl Chloride + Primary Amine → Secondary Amide (+ Alkyl Ammonium Chloride)

Carboxylic Acids

  • A carboxylic acid group can lose a proton to become a negatively charged carboxylate ion.








     

  • Carboxylic acids can be formed by the oxidation of a primary alcohol with acidified potassium dichromate, under reflux conditions.

  • Carboxylic acids can be reduced to primary alcohols using hydride ions (from lithium tetrahydridoaluminate (LiAlH )).

  • The reduction must be in dry ether as LiAlH  reacts violently with water molecules.

  • Negatively charged carboxylate ions can bond ionically with positive ions, forming carboxylate salts.
     

Carbonyls (reduction of)

  • A carbonyl group is a carbon double bonded to an oxygen atom (C=O).

  • Aldehydes have a carbonyl group at the end of a carbon chain; ketones have a carbonyl group in the middle of a carbon chain. 

  • Aldehydes can be formed from the oxidation of primary alcohols; ketones can be formed from the oxidation of secondary alcohols.

  • The slightly positive carbon in an aldehyde and a ketone reacts with nucleophiles in addition reactions.

  • Aldehydes can be reduced with hydride ions (H⁻) to primary alcohols; ketones can be reduced with hydride ions to secondary alcohols.

  • A common source of hydride ions for reduction is the compound lithium tetrahydridoaluminate (LiAlH  )

Identifying Carbonyls

  • Aldehydes can be oxidised to carboxylic acids. It is not possible to oxidise a ketone.

  • The ability to further oxidise an alcohol and not a ketone can be used to help distinguish between aldehydes and ketones.

  • Aldehydes can be identified by using Tollens’ reagent (silver precipitate formed) or Fehling’s solution (red precipitate formed).  

  • Aldehydes and ketones can both be identified by using Brady’s reagent, producing an orange precipitate.

  • A methyl carbonyl group can be identified using a (warm) mixture containing iodine and sodium hydroxide, forming a yellow solid.

Nucleopilic Addition of Carbonyls

  • Hydrogen cyanide reacts with carbonyl groups in nucleophilic addition reactions forming hydroxynitriles:







     

  • Hydrogen cycande is highly reactive (and toxic), it can be prepared ‘in situ’ using a metal cyanide (KCN or NaCN) and sulfuric acid (H SO ). 

  • The length of the carbon chain in a carbonyl molecule can be extended using this reaction

Esters

  • Esters are made from a carboxylic acid and alcohol held together by an ester link.

  • An ester is formed in an esterification reaction by reacting a carboxylic acid and alcohol together, in the presence of concentration sulfuric acid (under reflux conditions).

  • Esterification is a condensation reaction as water is released as a product.

  • Ester links can be broken in hydrolysis reactions (addition of water).

  • Hydrolysis of an ester in acidic conditions produces a carboxylic acid and alcohol.

  • Hydrolysis of an ester in alkaline conditions produces a carboxylate ion and alcohol.

  • Fats and oils are natural examples of esters, called tri-glycerides, made from glycerol (propane-1,2,3-triol) and long chain fatty acids (carboxylic acids with long carbon chains).​

Naming of Esters

  • Esters are named based on the length of the carbon chains in both the carboxylic acid and alcohol that are used to make the ester.

  • The general form is alkyl-carboxylate.




     

  • The first part of the name comes from the carbon chain in the alcohol (-alkyl).

  • The second part of the name is the same as the carboxylate ion of the carboxylic acid (-carboxylate).

Optical Isomerism

  • Carbon can form four covalent bonds, if a carbon atom is bonded to four different groups there are two possible ways of arranging the groups, creating two possible isomers







     

  • Each isomer is a mirror image of the other and both isomers rotate plan polarised light in opposite directions.

  • If equal amounts of both isomers are present in a mixture, there is no overal rotation of plane polarised light - the mixture is racemic.

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A2-Level Benzene, Aromatic Chemistry

Benzene Structure

  • Benzene is a cyclic hydrocarbon made of six carbon atoms and six hydrogen atoms (C H ).

  • The Kekule structure shows bonding in benzene as alternating carbon double bonds.

    • Bond length data and enthalpies of hydrogenation for benzene show this to be incorrect

  • Each carbon atom has an un-bonded electron in a p-orbital. The p-orbitals of all carbon atoms merge to create two rings of electron density - spreading electrons throughout the carbon ring (delocalised electrons)







     

  • Due to the delocalised electron rings, benzene molecules are easily attacked by electrophiles.

  • Compounds that contain benzene are called aromatic compounds.

    • The study of these compounds is called aromatic chemistry.

       

Benzene Reactions

  • Electrophiles (electron deficient species) are attracted to the ring of delocalised electrons in benzene, nucleophiles are repelled by the delocalised electrons.

    • Benzene reacts with strong electrophiles​

  • Electrophiles react with benzene in electrophilic substitution reactions, forming a bond with a carbon atom in the ring and forcing the hydrogen atom already bonded to the carbon atom to be lost.







     

Nitration of Arenes

  • The nitration of an arene (compound with a benzene ring in) is an electrophilic substitution reaction, with a nitronium ion, NO⁺ , acting as the electrophile.​

  • The nitronium ion is produced by reacting concentrated nitric acid with concentrated sulfuric acid:



     

  • The reaction is carried out at 55ºC and produces a nitroarene.

     

Acylation of Arenes

  • Acylation of benzene involves the substitution of an acyl group onto a benzene ring:









     

  • Acyl chlorides are reacted with a halogen carrier (AlCl ) to form an acyl ion, which acts as a strong electrophile and reacts with the benzene ring in an electrophilic substitution reaction. 

  • The reaction requires warm conditions, and HCl is formed as a product.

     

Alkylation of Arenes

  • Alkylation of benzene involves the substitution of an alkyl group onto a benzene ring.

  • Alkyl groups are carbon chains bonded to another carbon atom (i.e. methyl).











     

  • Halogenoalkanes are reacted with a halogen carried to produce an R group with a positive charge that can act as the required electrophile.

  • The reaction requires warm conditions, and HCl is formed as a product.

Bromination of Arenes

  • The bromination of benzene involves the electrophilic substitution of a bromine onto a benzene ring:








     

  • A halogen carrier is needed (usually AlBr ) to create an electrophile, as the delocalised electrons in the benzene are unable to polarize the bromine molecule sufficiently (unlike with bromine and alkenes).

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A2-Level Phenol

Phenol

  • Phenol is an organic molecule that has a benzene ring with a hydroxyl group attached:






     

  • The lone pair on the oxygen atom merges with the pi-bonding system in benzene, enabling the OH⁻ group to release a proton.

    • This means phenol can act as a weak acid.​

  • Alcohols are not weak acids as a negative charge on the oxygen atom is not stabilised.

 

Phenol Reactions

  • The hydroxyl group (OH) in phenol ‘activates’ the benzene ring, making it more reactive with electrophiles.

  • Phenol is able to react directly with bromine (unlike benzene) in multiple substitution reacts to produce 2,4,6-tribromophenol.

  • Nitration of phenol is easier than nitration of benzene. Only nitric acid (HNO ) at room temperature is required (for nitration of benzene: nitric acid, sulfuric acid and temperature of 55ºC).

  • The OH group in phenol has a directing effect, meaning substitutions are most likely to occur on the second, fourth and sixth carbon (carbon one is occupied by the OH group.

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A2-Level Amines and Amides

Amines

  • Amines contain a carbon chain bonded to a nitrogen atom that is bonded to either hydrogen or another carbon chain (alkyl).

  • Amines can be primary, secondary or tertiary (quaternary amines exist as positively charged ions). 

  • A lone pair of electrons on the nitrogen atom enables amines to act as bases (and nucleophiles).

  • Primary amines are less basic than secondary amines, and secondary amines are less basic than tertiary amines. 

  • This is due to the positive inductive effect of alkyl chains.

  • Aromatic amines are weak bases as the benzene ring is electron withdrawing.
     

Producing Primary Amines

  • Primary amines are formed by the reaction of a halogenoalkane with ammonia.







     

  • Ammonia acts as a nucleophile and is substituted for the halogen, making the mechanism nucleophilic substitution.

  • Ammonium ions are formed at the end of the reaction because the ammonia (not the halide ions) removes a proton from the positively charged nitrogen in the intermediate.

Producing Secondary and Tertiary Amines

  • If a halogenoalkane is reacted with an excess of ammonia, a primary amine is formed.

  • When insufficient ammonia is present to undergo substitution with all the halogenoalkane molecules, the primary amines that did form undergo further substitution and can become secondary, tertiary and quaternary amines.

  • Once four substitutions have happened, the nitrogen has four bonds and becomes positively charged. This quaternary ammonium ion is attracted to negative halide ions and a quaternary ammonium salt forms

Producing Aromatic Amines

  • Aromatic amines are formed by the reduction of a nitroamine:










     

  • A tin catalyst and concentrated hydrochloric acid are used under reflux conditions (reflux conditions are essential to stop gaseous vapors of hydrochloric acid being released). 

  • Acidic conditions result in an ammonium ion being formed, which is converted into an amine group by adding sodium hydroxide.

Amides

  • Amides are carboxylic acid derivatives that have a carbonyl group with an amine attached to the carbon from the carbonyl, they are generally unreactive.






     

  • Amides can be primary, secondary and tertiary. 

  • (Primary) amides are formed from acyl chlorides and ammonia in a nucleophilic addition-elimination reaction, producing an ammonium chloride salt.

  • Secondary amides are formed from acyl chlorides and a primary amine. 

  • Tertiary amides are formed from acyl chlorides and a secondary amine.

A2-Level Azo-Dyes

Organic Dyes

  • Organic dyes are compounds bonded to materials to give the material a specific colour.

  • Within organic dyes, electrons are excited from low energy to high energy by absorbing energy from the visible part of the electromagnetic spectrum.

  • The part of the dye where the excitation of electrons occurs is called the chromophore.

  • Most organic molecules absorb higher energy waves (ultraviolet light – not visible) to excite electrons. Increased levels of electron delocalisation decrease the energy required to excite electrons and enable visible light to be absorbed by the molecule, forming a chromophore. 

  • Wavelengths of light not absorbed by the chromophore are scattered by the dye and are observed as the complimentary colour to the colour of light absorbed by the dye.
     

Azo-Dye Formation

  • The chromophore in an organic dye requires a high level of electron delocalization.

  • Azo dyes are compounds formed from a diazonium salt and an aromatic compound (usually phenol)



















     

  • The formation of azo dyes happens in two stages:

  • A diazonium salt is formed from the reaction of phenylamine with nitrous acid and hydrochloric acid. 

  • The diazonium salt is coupled with the phenol by reacting them together with sodium hydroxide.

  • Diazonium salts are highly unstable and to stop them decomposing during the reaction, low temperatures are used (5ºC).

A2-Level Polymerisation

Addition Polymerisation

  • Polymers are long chain molecules made of repeating units bonded together.

  • Molecules that form repeating units are called monomers. 

  • Alkenes can undergo addition polymerisation to create polyalkenes.

    • The double carbon bond can be opened to enable each carbon from the double bond to form new bonds with another repeating unit. 
       

Condensation Polymerisation

  • Condensation polymers are formed when monomers bond together and one water molecule is released per bond.

  • To break apart a condensation polymer (into its monomers), a water molecule is needed and a hydrolysis (breaking of bond with water) reaction happens.

  • Polyamides are formed from the reaction of a di-carboxylic acid with a di-amine. The links between each monomer are called a peptide links.

  • Polyesters are formed from the reaction of a di-carboxylic acid with a di-ol in the presence of an acid. 

Amino Acids and Proteins

  • Amino acids are a collection of different organic molecules that all contain an amine group and a carboxylic acid group.








     

  • Amino acids join together to form proteins.

  • The amine group in an amino acid can accept a proton (basic) to become a positive ion.

  • The carboxylic acid group in an amino acid can donate a proton (acidic) to become a negative ion.

  • Different amino acids lose or gain protons at different pHs.

  • A zwitterion is an amino acid that has lost a proton from its carboxylic acid group but gained a proton at its amine group – forming negatively and positively charged groups, but no overall charge.

  • Zwitterions are formed at an amino acid’s isoelectric point.
     

Properties of Polymers

  • Longer chain polymers have higher melting points than shorter chain polymers.

  • The larger the contact surface between polymers, the greater the number of intermolecular forces that can form between them, resulting in a higher melting point.

  • Branched chain polymers have lower melting points than straight chained polymers as there is less contact surface between them, resulting in fewer intermolecular forces.

  • Addition polymers are not biodegradable.

  • Condensation polymers are biodegradable and are broken down in hydrolysis reactions.