Unit 5: Compounds containing the carbonyl Group

A carbonyl group is -CO-  (essentially C=O)

Aldehydes and ketones both contain carbonyl groups.

An aldehyde has a C=O at the end of it's longest chain of carbon atoms. (-CH2OH)

A ketone has a C=O anywhere else but at the end of its longest chain of carbon atoms.(-CH(OH)-)

Aldehydes are oxidised from primary alcohols and can be further oxidised to carboxylic acids.

RCH2OH + [O] --> H2O + RCHO         + [O] --> RCOOH

Ketones are oxidised from secondary alcohols.

RCHOHR + [O] --> RCHOR

R represents an alkyl or aryl (aromatic) group.

To oxidise a primary alcohol to a aldehyde, remember to distil.

To oxidise a primary alcohol to a carboxylic acid, remember to reflux.

When a primary or secondary alcohol is oxidised, the colour change is orange to green. (Assuming acidified potassium dichromate (IV) is used)

Tertiary alcohols are not easily oxidised. So acidified dichromate remains orange when added to a tertiary alcohol and heated. This is because there is no Hydrogen that can be easily removed from the carbon that is bonded to the -OH group (hydroxyl group). Hence, the oxidant [O] cannot remove two hydrogens, one attached to the carbon and one from the hydroxyl group to form water (2H + [O] --> H2O).

However, a much stronger oxidant can oxidise tertiary alcohols by breaking a C-C bond, this creates products with shorter chain lengths.

The carbonyl group in aldehydes and ketones reacts very similarily. Both C=O's can react by nucleophilic addition. However, aldehydes are reducing agents (an electron pair donor that goes onto being oxidised, in this case to a carboxylic acid) and ketones are not. 

The O in the C=O bond becomes negatively charged because it has a hihg electronegativity which resultss in the bond being polar and the carbon electron deficient. Carbon becomes a delta plus and the oxygen a delta minus which can attract electrophiles (positively charged substances).

This results in a hydroxynitrile.

Aldehydes are easily oxidised to carboxylic acids but ketones cannot be oxidised. We can use this fact to establish between aldehydes and ketones. Fehlings solution and tollen's reagent are both gentle oxidising agents that can be used.

Fehlings    Blue --> Brick-red indicates aldehyde

 RCHO     +    (2Cu)2+    +   4OH-    --->   RCOOH    +   CU2O    +   2H2O

Fehlings has a copper (II) complex ion (which is blue) in alkaline solution that when warmed with an aldehyde is reduced (and the aldehyde oxidised) to a brick-red precipitate of copper (I) oxide.

Tollens   Colourless--> silver mirror   indicates aldehyde

RCHO    +   2[Ag(NH3)2]+    +    2OH-   --->   RCOOH   +  2Ag   +   4NH3     +   H2O

The colourless solution of silver (I) complex ion has diamminesilver(I) ions reduced to metallic Ag which causes a 'silver mirror' to build up in the test tube. 

Oxidising Agent Primary Alcohol Secondary Alcohol   Tertiary Alcohol  Aldehyde  Ketone

K2Cr2O7 /H+               Yes                   Yes                              No                   Yes           No

Fehlings reagent           No                     No                               No                   Yes           No

Tollen's reagent             No                     No                              No                    Yes          No

Remember!

Tollens and Fehlings don't work on alcohols, even if they are primary or secondary.

Very hard to oxidise a tertiart alcohol.

Distil a primary alcohol to get an aldehyde. Reflux to get a carboxylic acid.

You can't further oxidise a carboxylic acid.

The oxidation reactions to make aldehydes and ketones can be reversed to reform the initial alcohl by using a strong reducing agent (a reducing agent loses electrons, an oxidising agent gains electrons) such as lithium tetrahydridoaluminate (LiAlH4, which is used in a non-aqueos solvent). 

As Oxidising agents are represented by [O], reducing agents are represented by [H]

RCOOH + 2[H] --> RCHO        + 2[H] -->   RCH2OH

Carboxylic acid --> Aldehyde --> Primary alcohol

RCOR + 2[H] --> RCHOHR

Ketone --> Secondary alcohol

REMEMBER TO USE 2[H] with these carbonyls

This reductions is by the nucleophilic addition mechanism, with the OH- as the nucleophile.

Carboxylic acids are formed when primary alcohols are oxidised in reflux past aldehydes. They have the functional group of COOH on an end carbon.

Carboxylic acids with fewer that six carbon atoms per molecule are water soluble. In water they become only slightly ionised.

RCOOH + H2O     =       RCOO-     +     H3O+    This is an equilibrium reaction.

This gives a low concentration of hydrogen and alkanoate ions.

As [H+] is small, the pH for carboxylic acids is not low (pH = -log[H+]) Thus carboxylic acids are weak acids.

You can test for the hydrogen ions by seeing if the solution displaces CO2 gas from an aqueous solution of Sodium carbonate. The CO2 product provides a useful indicator for the presence of a carboxylic acid. (CO2 turns lime water cloudy and puts out a lit splint).

2RCOOH    +     Na2CO3   --->   2RCOO-Na+    +   CO2   +    H2O

Esters are formed when an alcohol and a carboxylic acid react together in the presence of a strong acid catalyst (e.g. sulphuric acid or conc. HCl)

R1COOH    +      HOR2       =(H+ catalyst)=      R1COOR2       +   H2O

The C-O bond in the acid breaks rather than the C-O bond in the alcohol.

The ester formation reaction is called esterification or sometimes known as a condensation reaction  (as a small molecule, H2O, is eliminated from two carbon-based molecules that form a larger structure).

Esterification is also a reversible reaction and tend to produce low yields of esters due to the position of equilibrium. Higher yields of esters can be produced by reacting the alcohol with the acid anhydride or acid chloride instead of the carboxylic acid. These reactions are called acylations (see unit 7 on amines).

When naming amines, Remember to name the right bit, or the part after the -COO-, first and then the part to the left second. e.g. below is methyl ethanoate

Diferent esters have different fragrances and tastes, the artificial fruit flavours used in confectionaries can be made by mixing esters. In nature, the smells and tastes of things like fruit are thanks to esters.

Esters are also used as plasticisers, which are additives added to polymers to increase their flexibility.

Polychloroethane (PVC) is a strong and rigid polymer used in things such as drain pipes, but when treated with an ester become flexible cling film. 

VC has very long polymer chanins, the C-Cl bond is polar which increases the strength of intramolecular forces (dipole-dipole). An ester increases the distance between molecule of PVC by coming between them and weakening the intramolecular forces, the chains ofmolecules can then slide over each other more easily making PVC more flexible.

Hyrolysis of esters has the reverse effect of esterification. The molecule is split in two by water to reform the acid and the alcohol. The reaction requires heat and a conc. H2SO4 or NaOH (in excess or the carboxylic acid would become it's salt) catalyst.

Soaps are the salts of long-chain carboxylic acids produced by ester hydrolyisis. Fatty acids, also know as the carboxyilic acids, are derived from naturally occuring fats and oils  which are naturally occuring tri-esters of the trihydric (three OH groups) alcohol propane-1,2,3-triol (aka glycerol) and three fatty acids. These acids are normally around 14,16,18 or 20 carbon atoms long.

Glycerol + long chain carboxylic acids   =    triester   + water

The making of soaps is called saponificiation, as making esters is esterification. It requires boiling fats and acids in the presence of aqueous NaOH solution.

After boiling is complete, common salt is added to precipitate the soap.

Whenever soap is manufactored, glycerol is a useful byproduct in pharmaceutrical and cosmetic preparations. 

Soaps are also anionic detergents, the megatively charged carboxyl group is attracted to water: it is hydrophilic.