Chemistry Unit 4- Carbonyl Compounds

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  • Created on: 10-06-11 17:23

Aldehydes and Ketones

Both carbonyl compounds, so contain the carbonyl functional group C=O

The difference= have their carbonyl groups in different positions

Aldehydes have their carbonyl group at the end of the carbon chain. There names end in -al. e.g. methanal, propanal. Have the formula RCHO

Ketones have their carbonyl group in the middle of the carbon chain. There names end in -one, and often have a number to show which carbon the carbonyl group is on. e.g. propanone, pentan-2-one. Have the formula RCR'O. 

Tests to distinguish:

All work on the idea that an aldehyde can be easily oxidised to a carboxylic acid, but a ketone can't.

As an aldehyde is oxidised, another compound is reduced= so a reagent is used that changes colour as it's reduced. 

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Aldehyde and Ketone Tests

(The aldehyde/ ketone will be flammable, so heat the mixture in a water bath than over flame). 

Tollen's reagent 

Colourless solution of silver nitrate dissolved in aqueous ammonia.If it's heated in a test tube with an aldehyde, a silver mirror forms after a few mins.

Ag(NH3)2+ (aq and colourless) + e- --> Ag (s) + 2NH3 (aq)

Fehling's or Benedict's Solution:

Fehling's solution is a blue solution of silver nitrate dissolved in sodium hydroxide. If it's heated with an aldehyde the copper (II) ions are reduced back to a brick-red precipitate or copper (I) oxide:

Cu 2+ (blue and aq) + e- --> Cu+ (s and brick-red)

Benedict's is exactly the same except the copper (II) ions are dissolved in sodium carbonate instead. Still get brick- red precipitate

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Reduction back to Alcohols

Primary alcohols can be oxidised to produce aldehydes and carboxylic acids

Secondary alcohols can be oxidised to make ketones

Using a reducing agent you can reverse these reactions. NaBH4 (sodium tetrahydridoborate (III) or sodium borohydride) dissolved in water with methanol is usually the reducing agent used. 

In equations, [H] is often used to indicate a hydrogen from a reducing agent

They're nucleophilic addition reactions and the H- ion acts as a nucleophile and adds onto the carbon atom

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Alcohols and Hydrogen Cyanide Reactions:

Aldehyde to a primary alcohol:cccccccccccccccc Ketone to a secondary alcohol:







Hydrogen Cyanide reacting with Carbonyls:

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Hydrogen Cyanide...

Hydrogen cyanide reacts with carbonyl compounds to produce hydroxynitriles (molecules with a CN and an OH group).

It's a nucleophilic addition reaction- a nucleophile attacks the molecule, causing an extra group to be added.

Hydrogen Cyanide's a weak acid- it partially dissociates in water to form H+ ions and CN- ions. HCN <---> H+ + CN-

1. The CN group attacks the partially positive carbon atom and donates a pair of electrons. Both electrons from the double bond transfer to the oxygen.

2. H+ (from either hydrogen cyanide or water) bonds to the oxygen to form the hydroxyl group (OH)

Risk= Hydrogen cyanide is a highly toxic gas, to reduce risk, a solution of acidified potassium cyanide is used instead. The cyanide ions needed for the reaction are formed in the solution. Reaction is done in a fume cupboard and the compounds heated in  water bath or electric mantle as they are flammable. 

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Carboxylic Acids

They contain the carboxyl functional group -COOH (carbonyl and hydroxyl group)

To name- name the longest alkane chain, take off the "e" and add "-oic acid"

The carboxyl group is always at the end of the molecule, and its more important that other functional groups, so the other functional groups in the molecule are numbered starting from this carbon. 

Carboxylic acids are weak acids- in water they partly dissociate into a carboxylate ion and an H+ ion.

RCOOH <--> RCOO- + H+  

RCOOH = carboxylic acid and RCOO- = carboxylate ion

Equilibrium lies to the left because most of the molecules don't dissociate

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Reactions to form carbon dioxide

Carboxylic acids react with carbonates, CO3 2-  to form a salt, CO2 and H2O:

Ethanoic acid  sodium ethanoate

2CH3COOH(aq) + Na2CO3(s) --> 2CH3COONa(aq) + H2O(l) + CO2(g)

Carboxylic acids react with hydrogencarbonates (HCO3-) forming salt, CO2, H20:

Ethanoic acid sodium ethanoate

CH3COOH(aq) + NaHCO3(s) --> CH3COONa(aq) + H2O(l) + CO2(g)

In these reactions carbon dioxide fizzes out of solution

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Carboxylic acids react with alcohols to form ester

If you heat a carboxylic acid with an alcohol in the presence of a strong acid catalyst, you get an ester. Called an esterification reaction. Concentrated sulfuric acid is usually used as the acid catalyst.

Carboxylic acid + alcohol <--under reflux with H+--> easter + water

RCOOH + ROH <---> RCOOR + H2O    (oxygen in ester comes from the alcohol)

Its also a condensation reaction as it releases water 

How ethanoic acid reacts with ethanol to make ethyl ethanoate (an ester):

Ethanoic acid + ethanol <-----> ethyl ethanoate + water


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How they are named...

Ester is formed by reacting an alcohol with carboxylic acid.

Ester name is made up of two parts- 1. the alcohol and 2. the carboxylic acid

1. Look at the alkyl group that came from the alcohol:

CH3COOCH2CH3 - this is an ethyl group

2. Look at the part that came from the carboxylic acid. Awap the "oic" for "oate":

CH3COOCH2CH3- this came from ethanoic acid, it is ethanoate.  

3. Put the two together. It is ethyl ethanoate CH3COOCH2CH3

 The name is written the opposite way round from the formula. 

This goes for molecules with benzene rings too e.g. react methanol and benzoic acid and you get methylbenzoate.

If either of the carbon chains is branched you need to name the attached groups. For an ester, number starting from the C atoms in the C-O-C bond.

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Uses of Esters

1. Have a sweet smell- varying from gluey sweet for smaller esters to a fruity smell for larger ones. This makes them useful in perfumes. The food industry uses esters to flavour things like drinks and sweets.

2. Esters are polar liquids so lots of polar organic compounds will dissolve them. They have low boiling points , so they evaporate easily from mixtures. Making them good in glues and printing inks.

3. Esters are used as plasticisers- they're added to plastics during polymerisation to make the plastic more flexible. Over time, the plasticiser molecules escape though, and the plastic becomes brittle and stiff. 

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Acyl Chlorides

Acyl (for acid) chlorides have the functional group COCl

Their general formula is C n H 2n-1 OCl

All their ends end in -oyl chloride

In naming- the carbon atoms are numbered starting from the the one with the acyl functional group (same as with carboxylic acids).

Acyl chlorides easily lose their chlorine...

With water: vigorous reaction with cold water, producing carboxylic acid

CH3COCl + H2O ---> CH3COOH + HCl

ethanoyl chloride ---> ethanoic acid

With alcohols: vigorous reaction at room temperature, producing an ester


ethanoyl chloride ---> methyl ethanoate

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Acyl Chlorides

With ammonia: violent reaction at room temperature, producing an amide

CH3COCl + NH3 --> CH3CONH2 + HCl

ethanoyl chloride ---> ethanamide

With amines: violent reaction at room temperature producing N-substituted amide


ethanoyl chloride --> N-methylethanamide

Each time, Cl is substituted by an oxygen or nitrogen group, and misty fumes of hydrogen chloride are given off. 

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Acid Anhydrides

An acid anhydride is made from two identical carboxylic acid molecules. 

If you know the name of the carboxylic acid, take away the "acid" and add "anhydride"


2 ethanoic acid --> ethanoic anhydride

Need to know the reactions of water, alcohol, ammonia and amines with acid anhydrides. 

Almost the same as with acyl chlorides, the reactions are just less vigorous and a carboxylic acid is formed instead of HCl:

e.g. (CH3CO)2 O (l) + H2O (l) --> 2CH3COOH (aq)

 ethanoic anhydride + water --> ethanoic acid

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Acyl Chloride reactions are Nucleophilic Addition-

In acyl chlorides, both the chlorine and the oxygen atoms draw electrons towards themselves.

So the carbon has a slight positive charge- so its easily attacked by nucleophiles e.g. reaction between ethanoyl chloride and methanol:

Methanol is the nucleophile here. It attacks the partially positive carbon on the acyl chloride, and a pair of electrons from the C=O bond are transferred to oxygen. Now the pair of electrons on the oxygen reform the double bond and the chlorine's kicked off. The chlorine now bonds with the hydrogen in the hydroxyl group and the hydrogen chloride is now eliminated:




The other reactions of acyl chlorides all work in the same way. Just change the nucleophile to water (H2O:), ammonia (:NH3) or an amine (e.g. CH3:NH2)

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Manufacture of aspirin

Aspirin is an ester- it's made by salicylic acid (which has an alcohol group) with ethanoic anhydride or ethanoyl chloride.

Ethanoic anhydride is used in industry because:

* It's cheaper than ethanoyl chloride

* It's safer to use than ethanoyl chloride as it's less corrosive, reacts more slowly with water, and doesn't produce dangerous hydrogen chloride fumes. 

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