AQA A Level Chemistry复习笔记7.2.3 Nucleophilic Addition

Nucleophilic Addition

 

  • Many of the reactions which carbonyl compounds undergo are nucleophilic addition reactions
  • The carbonyl group -C=O, in aldehydes and ketones is polarised
  • The oxygen atom is more electronegative than carbon drawing electron density towards itself
  • This leaves the carbon atom slightly positively charged and the oxygen atom slightly negatively charged
  • The carbonyl carbon is therefore susceptible to attack by a nucleophile, such as the cyanide ion

The carbonyl group here has a dipole with a delta positive carbon and a delta negative oxygen

7.2.3-carbonyl-dipole

General Mechanism with an aldehyde:7.2.3-Nucleophilic-addition-with-aldehydesGeneral Mechanism with a ketone: 7.2.3-Nucleophilic-addition-with-ketones

In both reactions, the nucleophile (Nu) attacks the carbonyl carbon to form a negatively charged intermediate which quickly reacts with a proton

 

Addition of HCN to carbonyl compounds

  • The nucleophilic addition of hydrogen cyanide to carbonyl compounds is a two-step process, as shown below7.2.3-Ethanal-and-CN-Nucleophilic-addition
  • In step 1, the cyanide ion attacks the carbonyl carbon to form a negatively charged intermediate
  • In step 2, the negatively charged oxygen atom in the reactive intermediate quickly reacts with aqueous H+ (either from HCN, water or dilute acid) to form 2-hydroxynitrile compounds,
    • e.g. 2-hydroxypropanenitrile

     

Exam Tip

By convention, we write the formula of an ion then its charge, e.g. CN-.

  • The actual negative charge on the cyanide ion is on the carbon atom and not on the nitrogen atom.
  • However, when writing it together as :CN- you will not be penalised for writing the minus charge after the N.
  • This reaction is important in organic synthesis, because it adds a carbon atom to the chain, increasing the chain length
  • The products of the reaction are hydroxynitriles
    • The nitrile group is the priority functional group so it is attached to carbon 1 and results in the suffix -nitrile
    • The hydroxyl group is not the priority functional group so the hydroxyl group is named using the hydroxy- prefix, rather than the -ol suffix

     

Forming Enantiomers

Forming Enantiomers

  • The reaction mixture which is produced will be a racemic mixture
    • There will be a 50:50 mixture of both enantiomers, because there is a 50:50 chance of attack happening on each side
    • Racemic mixtures are formed when addition reactions are done with a planar starting material, because the reaction takes place with equal probability from either side of the plane7.2.3-Attack-of-carbonyl-from-both-sides

     

The attack from the :CN- has a 50:50 chance of taking place on either side of the C=O bond

 

7.2.3-Hydroxynitrile-enantiomers

A racemic mixture, or racemate, of each enantiomer is formed

 

  • The enantiomers in a racemic mixture both rotate plane polarised light, but they rotate it in opposite directions
  • Because there is a 50:50 mixture of both enantiomers, each rotating light in equal amounts but opposite directions, the effects on plane polarised light are cancelled out
  • Therefore, there will be no effect on plane polarised light with a racemic mixture
    • The optical rotation of the racemic mixture is zero

     

  • This can be used as a test to determine whether a mixture is racemic
    • If you know that a sample contains enantiomers of chiral compounds, and when tested there is no effect on plane polarised light, then the reaction mixture must be racemic
    • If there is an effect on plane polarised light, then the sample is not racemic

 

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