lecture 23 - Structure & Classification Amines are...

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Unformatted text preview: Structure & Classification Amines are classified as 1, 2, or , 3 amines: amines in which 1, 2, or 3 2 3 hydrogens of NH3 are replaced by alkyl or aryl groups Amines Chapter 23 Structure & Classification Amines are further divided into aliphatic, aromatic, and heterocyclic amines: aliphatic amine: an amine in which nitrogen is bonded only to alkyl groups aromatic amine: an amine in which nitrogen is bonded to one or more aryl groups (See next slide) Aromatic Amines p.899 Structure & Classification heterocyclic amine: an amine in which nitrogen is one of the atoms of a ring Structure & Classification Example: classify each amino group by type (1, 2, 3, aliphatic, aromatic, heterocyclic) CH3 H (a) N H (S)-Con iine (b) H N CH3 (c) O Cocain e O C6 H5 N COOCH3 N (S)-N icotine Pyrrolidine Piperid ine (h eterocyclic alip hatic amines) Pyrrole Pyridine (h eterocyclic aromatic amin es) 1 Nomenclature Aliphatic amines: replace the suffix -e of the e parent alkane by -amine amine Practice NH2 NH2 2-Propanamine H2 N NH2 (S)-1-Phen ylethanamine 1,6-Hexanediamine p.900 Nomenclature The IUPAC system retains the name aniline Nomenclature Among the various functional groups discussed in the text, -NH2 is one of the lowest in order of precedence H2 N OH OH NH2 (S )-2-Amin o-3-methyl1-bu tanol H2 N COOH 2-A min oeth anol 4-A minoben zoic acid Nomenclature Common names for most aliphatic amines are derived by listing the alkyl groups bonded to nitrogen in one word ending with the suffix amine Practice H CH3 NH 2 NH2 N Et 3 N Meth ylamine tert -Butylamine D icyclopen tylamine Triethylamin e p.901 2 Nomenclature When four groups are bonded to nitrogen, the compound is named as a salt of the corresponding amine These are classified as 4 ammonium ions Cl + NCH2 ( CH 2 ) 1 2 CH3 - Me4 N Cl + - Ph CH2 NMe 3 OH Benzyltrimeth ylammoniu m hydroxid e - Tetramethy Tetrad ecylpyrid inium ch loride ammonium (Cetylp yridin ium chloride) chloride p.902 Chirality of Amines Practice if we consider the unshared pair of electrons on nitrogen as a fourth group (lowest priority), then the arrangement of groups around N is approximately tetrahedral the u nsh ared electron pair in the fourth sp 3 hybrid orbital H Me N Et Et N Me H (S)-Ethylmethylamin e (R)-Ethylmethylamin e p.902 Chirality of Amines an amine with 3 different groups bonded to N is chiral and exists as a pair of enantiomers and, in principle, can be resolved p.903 3 Chirality of Amines in practice, however, they cannot be resolved because they undergo pyramidal inversion, which converts one enantiomer to the other Un hybridized 2p orbital sp 3 N H Et Me S Enan tiomer H Me N sp 3 Et N sp 3 The trans ition state has a p lane of s ymmetry; it is achiral R Enan tiomer p.903 Chirality of Amines pyramidal inversion is not possible with quaternary ammonium ions, and their salts can be resolved Physical Properties Amines are polar compounds, and both 1 and 2 amines form intermolecular hydrogen bonds Cl N - ClEt Me R Enan tiomer N Et Me S Enan tiomer N-H- - -N hydrogen bonds are weaker than OH- - -O hydrogen bonds because the difference in electronegativity between N and H (3.0 - 2.1 =0.9) is less than that between O and H (3.5 2.1 = 1.4) Physical Properties CH3 CH3 MW (g/mol) bp (C) -88.6 30.1 CH3 NH2 31.1 -6.3 CH3 OH 32.0 65.0 Fig. 23.1, p.904 4 Table 23.1, p.906 Table 23.1, p.906 Basicity All amines are weak bases, and aqueous solutions of amines are basic Basicity It is common to discuss their basicity by reference to the acid ionization constant of the conjugate acid So, the greater the pKa of the acid, the greater the basicity of the amine + H CH3 -N + H-O-H H + CH3 -N-H O-H H Methylammoniu m h yd roxide H Methylamine CH3 NH3 Ka = + H2 O + + CH3 NH2 + H3 O+ = 2.29 x 10-1 1 pK a = 10.64 [ CH3 NH2 ] [H3 O ] [CH3 NH3 ] Table 23.2, p.907 Table 23.2, p.907 5 Basicity using values of pKa, we can compare the acidities of amine conjugate acids with other acids Basicity-Aliphatic Amines BasicityAliphatic Amines Amine Ammon ia Primary Amines methylamin e ethylamine cyclohexylamine Secon dary A min es dimethylamine dieth ylamine Tertiary Amines trimeth ylamine triethylamin e Structure NH 3 CH3 NH2 CH3 CH2 NH 2 C6 H1 1 NH2 p Ka 9.26 10.64 10.81 10.66 pK b 4.74 3.36 3.34 3.19 CH 3 NH2 + CH3 COOH pK a 4.76 (s tronger acid) CH3 NH3 + CH3 COO pK a 10.64 (w eak er acid ) + - pK eq = -5.88 Ke q = 7.6 x 105 (CH3 ) 2 NH 10.73 (CH3 CH 2 ) 2 NH 10.98 (CH3 ) 3 N (CH3 CH 2 ) 3 N 9.81 10.75 3.27 3.02 4.19 3.25 All aliphatic amines are slightly stronger bases than ammonia because... They also have about the same base strength Basicity-Aromatic Amines BasicityAmine Aromatic A min es An iline 4-Methylanilin e 4-Ch loroaniline 4-N itroanilin e CH3 Cl O2 N NH2 NH2 NH2 NH2 4.63 5.08 4.15 1.0 Structure pK a of Conju gate A cid Heterocyclic Aromatic A min es Pyrid ine Imidazole N H p.907 N N 5.25 6.95 Basicity-Aromatic Amines Basicity aromatic amines are considerably weaker bases than aliphatic amines NH3 + OHCyclohexylammonium hydroxid e NH3 + OHA niliniu m h yd roxide pK a = 4.63 Basicity-Aromatic Amines BasicityAromatic amines are weaker bases than aliphatic amines because of two factors resonance stabilization of the free base, which is lost on protonation H N H H+ H N H + H N H + H N NH2 + H2 O Cycloh exylamine pK a = 10.66 NH2 + H2 O A niline H . . . . H . .. un hybridized 2p orb ital of N H H nitrogen is sp 2 h yb rid ized N H H . H 6 Basicity-Aromatic Amines Basicity- the greater electron-withdrawing inductive effect of the sp2-hybridized carbon of an aromatic amine compared with the sp3hybridized carbon of an aliphatic amine also decreases basicity p.909 Basicity-Aromatic Amines BasicityElectron-releasing, such as alkyl groups, increase the basicity of aromatic amines Basicity-Aromatic Amines Basicity 4-nitroaniline is a weaker base than 3-nitroaniline O2 N Electron-withdrawing groups, such as halogens, the nitro group, and a carbonyl group decrease the basicity of aromatic amines by a combination of resonance and inductive effects NH2 3-N itroanilin e pK a 2.47 O2 N NH2 4-N itroaniline pK a 1.0 Basicity-Aromatic Amines Basicity 4-nitroaniline is a weaker base than 3-nitroaniline Basicity-Aromatic Amines BasicityHeterocyclic aromatic amines are weaker bases than heterocyclic aliphatic amines N N H Piperidine p Ka 10.75 N Pyridin e pK a 5.25 N H Imid azole pK a 6.95 7 Basicity-Aromatic Amines Basicity in pyridine, the unshared pair of electrons on N is not part of the aromatic sextet Basicity-Aromatic Amines Basicity- pyridine is a weaker base than heterocyclic aliphatic amines because the free electron pair on N lies in an sp2 hybrid orbital (33% s character) and is held more tightly to the nucleus than the free electron pair on N in an sp3 hybrid orbital (25% s character) Basicity-Aromatic Amines BasicityImidazole Basicity-Guanidine BasicityGuanidine is the strongest base among neutral organic compounds This electron pair is not a part of the aromatic sextet : N + H2 O Aromaticity is maintained when imidazole is protonated H N+ : NH H2 N C NH2 Guan idine NH2 + + + H2 O OH Guanid inium ion H2 N C NH2 - p Ka = 13.6 This electron pair is a part of the aromatic sextet + OH - its basicity is due to the delocalization of the positive charge over the three nitrogen atoms + H2 N : : : N H2 : C N H2 N H Imidazole Which is the stronger acid? : N H Imidazolium ion H2 N + N H2 : C N H2 H2 N : N H2 C N H2 + Three equivalent contributing structures Reaction with Acids All amines, whether soluble or insoluble in water, react quantitatively with strong acids to form water-soluble salts OH HO NH2 + HCl HO (R)-N orepinep hrine (on ly s ligh tly s olu ble in w ater) H2 O HO HO (R)-N orepinep hrine h yd roch loride (a w ater-s olu ble salt) OH NH3 Cl + - p.912 8 Figure 23.2, p.915 Figure 23.2, p.915 Skip 23.7, 23.8 p.916 Hofmann Elimination Hofmann elimination: thermal decomposition of a quaternary ammonium hydroxide to give an alkene Step 1: formation of a 4 ammonium hydroxide CH3 I + CH2 -N- CH 3 + Ag 2 O H2 O Hofmann Elimination Step 2: thermal decomposition of the 4 ammonium hydroxide CH2 -N - CH 3 CH3 (Cyclohexylmethyl)trimethylammonium hydroxide CH2 + ( CH3 ) 3 N + H 2 O OH160 + CH3 CH3 (Cyclohexylmethyl)trimethyl- Silver oxide ammonium iodide CH3 OH+ CH2 -N- CH 3 + AgI Methylenecyclohexane Trimethylamine CH3 (Cyclohexylmethyl)trimethylammonium hydroxide 9 Hofmann Elimination Hofmann elimination is regioselective - the major product is the least substituted alkene CH3 + Hofmann Elimination HO H C H CH3 CH 2 C H H + N ( CH3 ) 3 HOH heat - E2 reaction (concerted elimination) H CH3 CH 2 H C C H N ( CH3 ) 3 N(CH3 ) 3 OH CH2 + (CH3 ) 3 N + H2 O Hofmann's rule: any -elimination that occurs Hofmann' preferentially to give the least substituted alkene as the major product is said to follow Hofmann's rule the regioselectivity of Hofmann elimination is determined largely by steric factors, namely the bulk of the -NR3+ group hydroxide ion preferentially approaches and removes the least hindered hydrogen and, thus, gives the least substituted alkene bulky bases such as (CH3)3CO-K+ give largely Hofmann elimination with haloalkanes Skip 23.10 Amines End Chapter 23 10 ...
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This note was uploaded on 09/06/2009 for the course CHEM chem 12 AB taught by Professor Valentin during the Spring '09 term at Evergreen Valley.

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