Amines Flashcards
What are amines
Derivatives of ammonia in which one or more of the H atoms in the ammonia molecule have been replaced by alkyl or aryl groups.
Physical properties of amines
Keep the basic shape and bond angles of ammonia (107). Hydrogen bonds in primary and secondary amines are not as strong as those in alcohols as N is less electronegative than O so not as high boiling points as comparable alcohols. Primary amines up to 4 carbons dissolve in water and alcohols. Most amines are soluble in less polar solvents.
Why are amines reactive?
They have a lone pair of electrons on the nitrogen which can be used to form a bond with a H+ ion (is a proton acceptor so Brønsted-Lowry base). The lone pair can also bond with an electron deficient carbon atom then the amine acts as a nucleophile.
What effect do alkyl groups have on the base strength of amines?
Alkyl groups release electrons towards towards the nitrogen atom (inductive effect). This increases the electron density on the nitrogen atom and therefore makes it a better electron pair donor (more attractive to protons). So secondary alkylamines are stronger than primary which are stronger than ammonia. Tertiary alkylamines not stronger than secondary as they are less soluble in water.
What effect do aryl groups have on the base strength of amines?
They withdraw electrons from nitrogen atom because the lone pair overlaps with the delocalised system on the benzene ring. Therefore phenylamine is weaker than even ammonia.
What is formed in nucleophilic substitution starting with ammonia?
Primary amine, secondary amine, tertiary amine, quarternary ammonium salt. Large excess of ammonia gives better yield of primary amine.
Roles of ammonia in nucleophilic substitution
First step it acts as a nucleophile. Second step it acts as a base.
How primary amines are made using reduction of nitriles
Haloalkanes react with cyanide ion in aqueous ethanol (room temperature) through nucleophilic substitution to form nitrile. Nitriles can be reduced to primary amines using H2 and nickel catalyst (needs 2 hydrogen molecules). Or can be reduced using LiAlH4 in dry ether (reacts vigorously with water).
How to make phenylamine
Nitrobenzene formed by electrophilic substitution. This is reduced to phenylamine using tin and HCl as the reducing agent (room temperature). The tin and HCl react react to form hydrogen which reduces the nitrobenzene by removing the O atoms in the NO2 group and replacing them with H atoms. Equation written using 6[H] and forms 2 water molecules. NaOH then added to liberate the free amine from the salt formed as the reaction is in HCl.
Mechanism for formation of amides
Addition elimination. Involves amine and acyl chloride. Arrow from lone pair on N of amine to delta- carbon and arrow from one of C=O bonds to O. Amine is attached to carbon and + on N and lone pair and - on O. Arrow from lone pair on O to C-O bond and arrow from C-Cl bond to Cl. Cl has lone pair and -. Arrow from N+-H bond to N+. Forms amide and separate H+.
Uses of amines
Used in manufacture of nylon, polyurethane, dyes and drugs. Quarternary ammonium compounds used in manufacture of hair and fabric conditioners. Wet hair and fabric pick up up negative changes on surfaces so positive charges of cations from the salt attract them to wet surface and form coating that prevent build up of static electricity keeping surface smooth.
