Ammonia and Amines threefold axis of rotation 111 107 Four sp 3 hybridized orbitals. Three used for bonding and one for the lone pair of electrons. trigonal pyramidal primary 1 secondary 2 tertiary 3 quaternary ammonia ethylamine diethylamine triethylamine tetraethylammonium ote that often can carry a formal charge of 1. Potentially chiral but "umbrella inversion" occurs rapidly in most cases i.e. the two enantiomers rapidly interconvert Bond Dipoles Molecular Dipole ydrogen Bonding as an -bond donor as an -bond acceptor Forms hydrogen bonds (but weaker than those of water). as a hydrogen bond donor is important in protein secondary structures
Acidity and Basicity of Amines Brönsted Bases Amines act as proton acceptors and are therefore Brönsted bases. Lewis Bases 2 base 3 conjugate Amines act as electron pair donors and are therefore Lewis bases. 3 3 3 BF 3 2 3 BF 3 3 The pk a of an can be used to estimate the base strength of the conjugate base. For example since the pk a of an alkylammonium ( 3 ) ion is 10 to 11, we see that this is somewhat more ic than water ( 2 ) but less ic than the hydronium ion ( 3 ). If we talk about the conjugate bases this is equivalent to saying that the alkylamine is more basic than water, but less basic than hydroxide. The pk a values (relative to water) and relative positions of the equilibria are shown below. pk a = -1.74 3 2 s pk a = 10.5 3 2 conjugate bases 2 pk a = 15.74 Because ammonia, 3, can act as both an and a base we say it is ambiphilic. ther ambiphilic compounds include water, alcohols, thiols and phosphines. a weak a very weak no ic properties at all pk a pk a 9.24 38 4 3 2 no basic properties at all a weak base a strong base
Effect of alkyl groups on base strength ammonia ethylamine diethylamine triethylamine tetraethylammonium less basic more basic not basic Alkyl groups are electron donating and therefore raise the energy of the lone pair. This effect is relatively small and there is not a huge difference in the actual basicity between 2 and 3. Some useful pk a values to remember B B strong s F SBF 5 < -12 2 S 4-10 l 3-7 -1.4 2 4.5 4 9.24 3 3 10.33 15.2 2 17 3 Ph 38 43 4 48 strong conjugate bases ucloeophilicity of Amines Many bases are also good nucleophiles. Amines are easily alkylated, indeed, it is difficult to stop alkylating since there is always a lone pair present to react. 3 3 I 3 I 3 I 3 I 3 2 ( 3 ) 2 ( 3 ) 3 ( 3 ) 4
ther itrogen ompounds Multiple Bonding at itrogen itrogen, like carbon, can hybridize orbitals to allow for π-bonding. sp 2 ybridization 3 3 an alkene 3 3 an imine 2 Pt 2 Pt 3 3 an alkane 3 3 a secondary amine The reduction of an imine yields an amine. sp ybridization 3 a cyanide or nitrile 3 3 a carboxylic The hydrolysis of a cyanide yields a carboxylic. Some representative bond energies 414 kj/mol 389 kj/mol 347 kj/mol 305 kj/mol 610 kj/mol 615 kj/mol 837 kj/mol 891 kj/mol
Imines 1. The alone pairof the amine attacks positive end of the carbonyl to give a tetrahedral intermediate 3. The tetrahedral intermediate decomposes with the loss and gain of a proton 3 3 3 2 3 3 3 3 3 3 3 3 3 A ketone A primary amine An aminol An imine 2. The protons are lost or gained to obtain a neutral species Iminium ions and enamines The presence a second alkyl group modifies the reaction slightly since there is no proton to be lost from the nitrogen in the third step. The iminium ion is formed instead, which normally tautomerizes to the enamine. 3 3 3 2 3 3 2 3 3 3 3 2 3 3 3 A ketone A secondary amine An aminol (cf hemiacetal or hemiketal) 3 3 3 3 An iminium ion 3 3 3 3 An enamine
Enamines are good carbon nucleophiles and are therefore good for - bond formation. 3 3 3 An enamine 3 3 Br 3 3 3 3 3-3 3 3 3 A new enamine 3 hydrolysis 3-2 3 3 3 eductive Amination 3 3 2 3 3 3 3 2 Pd/ or ab 4 3 3 3 A ketone A primary amine An imine A secondary amine yanides Alkylation of cyanides and subsequent reduction Br 2 Pt 2 a primary amine Alkylation of cyanide and subsequent hydrolysis Br 3 a carboxylic
Gabriel Phthalimide Synthesis pthalimide 1. K 2. 3 Br 3 3. 2 2 2 3 a primary amine chanism Alkylation of the phthalimide K K 3 Br 3 Decomposition of the phthalimide using hydrazine. 3 2 2 2 3 2 3-3 3 2 3