Ketone Functional Group
The functional group of the ketones is -CO-. The carbon atom is double bonded to the oxygen. The groups connected to the remaining bonds are alkyl or aryl groups (not hydrogen, which would result in the formation of an aldehyde). The alkyl or aryl groups modify the behaviour of the ketone in the normal way; for example as the molecular mass increases the boiling point of the resultant ketone increases. One of the most common ketones is acetone.
The functional group is prepared in one of two ways, either by partial chemical oxidation of a secondary alcohol or by dehydrogenation, with heat and a catalyst, of a secondary alcohol. Both of these processes remove the hydrogen from the hydroxyl group and allow a double bond to form between the connecting carbon and the oxygen.
The majority of reactions in which the ketone functional group takes place are addition reactions. Secondary alcohols can be readily recovered from ketones by breaking the double bond between the oxygen and carbon and adding hydrogen. This process can be carried out by adding the ketone to hydrogen in the presence of a catalyst or by using nascent hydrogen produced by the action of a dilute acid on a metal. A slightly different reaction occurs across the functional group if amalgamated zinc and concentrated hydrochloric acid are used. The carbonyl group is completely reduced to a -CH2 group. This process produces an alkane and is named the Clemmensen reaction after its discoverer (1913). The carbonyl group can be broken down by the action of hydrogen cyanide; a hydroxyl group and a nitrile group are formed in this reaction. The resultant chemical is a ketone cyanhydrin. A similar reaction occurs with sodium hydrogen sulfite to give ketone hydrogen sulfite compounds. The ketone functional group can also be involved in condensation reactions. In this type of reaction hydrazine (H2NNH2) removes the oxygen from the carbonyl. This oxygen reacts with hydrogen from the hydrazine to give water as an elimination product. The result is a hydrazone, which has the new functional group -CNNH2, with a double bond between the carbon and the first nitrogen. Derivatives of hydrazine react in a similar manner as do hydroxylamine (H2NOH) which gives oximes and semicarbazide (H2NNHCONH2) which gives semicarbazones.
The ketone functional group can also take part in autocondensation reactions which eliminate water. These reactions will produce an aromatic molecule from a starting point of an aliphatic, straight chain molecule. Polymerization across the carbonyl group is a very uncommon reaction, particularly compared to the similar group in aldehydes. This reaction is very similar to the aldol reaction.
To identify the presence of a ketone functional group several tests must be carried out. First, the compound under test is dissolved in methanol to which is then added a chemical called Brady's reagent. A yellow or orange precipitate shows the presence of the carbonyl group. The precipitate is then collected and dried and further tests can then be carried out to determine if the starting material was an aldehyde or ketone. Aldehydes will show a reaction with the following substances whereas ketones will not - Fehling's soluiton, Schiff's reagent, and Tollen's reagent.
The ketone functional group is the carbonyl group with its properties modified by the presence of two alkyl or aryl groups. The properties of the ketone group are very similar to those of the aldehyde group which consist of a carbonyl group, one alkyl or aryl group and a hydrogen atom.
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