Amine
Introduction Amines are derivatives of ammonia in which one or more hydrogen atoms have been replaced by alkyl Group. Amines are organic compounds and a type of functionl group that contain nitrogen as the key atom. Structurally amines resemble ammonia, wherein one or more hydrogen atoms are replaced by organic substituents such as alkyl and aryl groups. As displayed in the images below, primary amines arise when one of the three hydrogen atoms in ammonia is replaced by an organic substituent. Secondary amines have two organic substituents bound to nitrogen together with one hydrogen. In tertiary amines all the three hydrogen atoms are replaced by organic substituents.
Similarly, an organic compound with multiple amino groups is called a diamine, triamine, tetraamine and so forth. Nitrogen can take three or four bonds. When nitrogen takes four bonds, it has a positive charge. Nitrogen also carries a lone pair of electron. General characteristics of amines are 1. They act as nucleophile as a lone pair of electron attacks a positive charge. 2. When nitrogen forms forth bond, it becomes positively charged.
3. Amines are Lewis bases as they are capable of donating electron pair. 4. Donation of electron pair tends to stabilize carbocation in the same molecule. Basicity of Amine Amines are basic in nature. It is because they posses a lone pair of electron on nitrogen. They are weak bases. Electron withdrawing substituent reduces the basicity while electron donating group increases its basicity. The reduction in basicity is also possible due to presence of bulky functional group which reduces the ability of amine to donate lone pair (because of steric hindrance). The decreasing order of basicity is with electron donating group is 20 > 10 > NH3 An amine group attached to aromatic ring further reduces its basicity and aromatic amines are weaker base than corresponding aliphatic amines. The cause of this low basicity is delocalization of electron pair around aromatic ring. An electron withdrawing group from benzene further weakens aromatic ring basicity. Physical Properties of Amines Hydrogen bonding significantly influences the properties of primary and secondary amines as well as the protonated derivatives of all amines. Thus the boiling point of amines is higher than those for the corresponding phosphines but generally lower than the corresponding alcohols. Methyl-, dimethyl-, trimethyl-, and ethylamine are gases under standard conditions, while diethylamine and triethylamine are liquids. Most other common alkyl amines are liquids; high molecular weight amines are, of course, solids. Most aliphatic amines display some solubility in water, reflecting their ability to form hydrogen bonds. Solubility decreases with the increase in the number of carbon atoms, especially when the carbon atom number is greater than six. Aliphatic amines display significant solubility in organic solvents, especially polar organic solvents. Primary amines react with ketones such as acetone, and most amines are incompatible with chloroform and carbon tetrachloride. Tertiary amines of the type NHRR' and NRR'R" are chiral: the nitrogen atom bears four distinct substituents counting the lone pair.
Condensation with ketones Amine reacts with aldehyde or ketone by nucleophilic addition. If the amine is primary the initial addition product undergoes dehydration to form compound containing carbon-nitrogen double bond.It is called imine.
Aldehyde and ketone react with secondary amine to form compound called enamines. The general reaction is as follows.
Excess of acid should be avoided as amine will initially be protonated. The protonated amine becomes poor nucleophile due to positive charge and will not proceed further. The imine product exists as a tautomer with its corresponding enamine.
Wolff-Kishner Reduction The Wolff-Kishner reduction is a chemical reaction that fully reduces a ketone (or aldehyde) to an alkane.
The ketone or aldehyde can be reduced by treating aldehyde or ketone with hot acid in presence of amalgamated zinc. Some aldehyde or ketone do not respond to this process of reduction and in such cases Wolf- Kishner reduction is used. The reaction takes place in strongly basic medium and can be used for those compounds which are sensitive to acid.The first step in the reaction is formation of hydrazone.
Hydrazone is formed using the same mechanism which was used for preparation of imine. The only difference is the use of hydrazine in place of amine. The step is nucleophilic addition. A strong base is then added. The strong base brings tautomerization to a derivative with the structure – CH – N = NH. The derivative then undergoes the base catalyzed elimination of a molecu