Chemistry of Amines Ch. 24 of McMurry Classification: NH3= ammonia NR4

+ X- = quaternary ammonium salt (tetraalkylammonium)

NH2R = 1o NHR2 = 2o NR3 = 3o IR: N – H stretch: 2 bands 1 band no N-H, but adding

3250 – 3550 cm-1 ~3400 cm-1 HCl ammonium 2600 cm-1 N – H bending: 1580 – 1650 1515 cm-1 C – N stretch: weak, 1020 – 1250 cm-1

NMR: Deshielding effect of N shifts N-methyl to 2.42 ppm in 1H NMR 20 ppm downfield shift of most groups in 13C NMR

Mass spec: Odd-number of N atoms in a molecule produces an odd ion mass; molecules w/ just C, H, O always give even masses.

α-cleavage: RCH2 – CH2 – NR2 RCH2. + [CH2=NR2]+ Nomenclature Primary amines: 1) Hydrocarbon portion is named with “amine” added as a suffix 2) When other FG present, name “amino” group as substituent 2o and 3o: 1) If symmetrical, use prefix “di” or “tri” before alkyl group name 2) Unsymmetrical: smaller groups are named as N-substituents Aromatic amines: Named as derivatives of aniline with o, p system or with amino group as C1

Heterocyclic amines, common in nature have specific skeletons Nicotine and cocaine are examples of plant-derived amines, or “alkaloids” Cocaine is a “piperidine alkaloid”; nicotine contains two heterocycles

Properties of amines 1) Physical: Polarity: Due to polarity of N – H bonds, 1o and 2o amines can hydrogen-bond Elevated boiling points: amines > alkanes Water-solubility: 5C or smaller amines and ammonium salts are highly soluble, aromatic are moderately soluble Odor: fishy or rotten smell; diamines form from lysine in rotting flesh (cadaverine) Pharmacological effects: Many amines resemble amine neurotransmitters and affect nervous system functioning by mimicking or blocking these NTs 2) Chemical: Basicity and nucleophilicity Due to lone pair of e- on N, amines are good electron-donors (nucleophilic) and can accept protons (basic) R3N: + H – Cl NR3H+ Cl- Relative basicities of amines are expressed based on the acidity of their conjugate acids (the ammonium ions) [amine] [H3O+] Kb = Kw/Ka NR3H+ + H2O NR3 + H3O+ Ka = [ammonium salt] Rule: The lower the pKa of the ammonium ion (more acidic), the weaker the base

How do substituents affect basicity? Nonaromatic amines: basicity isn’t affected much by the nature of alkyl groups Aromatic (anilines) & heterocyclic amines are much less basic than alkylamines 1) The hybridization of the N atom: In heterocyclic aromatic amines the lone pair of e- may be part of the aromatic pi electron system as in pyrrole, or held tightly in an sp2 orbital as in pyridine.

2) Resonance-stabilization of anilines Arylamines (anilines) have delocalization of the N lone e- pair over the aromatic ring, and there is therefore a greater loss of stability on protonation to the arylammonium ion.

Substituent effects: See Table 24.2 E- donating groups on a ring increase basicity (alkyl, OR, OH, other NH2 groups) EWG decrease basicity (NO2, CN, CHO, Br, Cl)

Preparation of Amines 1) Reduction of nitriles or amides using LiAlH4 gives 1o amines

H3C

H2C

BrNaCN

H3C

H2C

C N1) LiAlH4, ether

2) H2O H3C

H2C

CH2

NH2

CH2

CNH2

O

H3C1) LiAlH4

2) H2O CH2

H2C

NH2

H3C, ether

2) Anilines can be prepared by reduction of nitrobenzenes 3) SN2 reactions with alkyl halides Use of ammonia as Nu gives a mixture of mono, di, tri- and even tetra-substituted amines (lesser amount of 3o amines, ammonium salts)

Azide synthesis: ONLY makes 1o amines…but sometimes with a bang!

Gabriel amine synthesis: 1o amino group using an imide as nucleophile

• The acidic N of phthalimide can be deprotonated much like a β-diketone • Alkylation occurs when treated with an alkyl halide • Addition of base hydrolyzes the imide, releasing the amine as a product:

aq. NaOH

H3C

H2C

BrNH3

H3C

H2C

NH2 H3C

H2C

NH

H2C

CH3H3C

H2C

N

H2CCH3

H2C

CH3

CH2Br

H2C N N NH CH2

NH2

NaN3 LiAlH4, ether

H2O

NO2

1) SnCl2, H3O+

2) NaOH

or H2, PtNH2

4) Reductive amination of aldehydes & ketones: 1o, 2o & 3o amines

Raney Ni = fine Ni particles with adsorbed H2 Another common reducing agent for this reaction is NaBH3CN in CH3OH 5) Rearrangements: Decarbonylation of amides to make 1o amines Work well for both alkyl and aryl amines: Hofmann rearrangement uses base to brominate the amide, followed by a migration of the R group to the nitrogen (Fig 24.5) In the Curtius rearrangement, an acyl azide forms, rearranges to isocyanate by migration of R, then decarbonylates Many common amine stimulant drugs are structurally similar to, and raise the levels of natural neurotransmitters including norephinephrine and dopamine. Some drugs prepared by the methods shown here include: Amphetamines (reductive amination) “Fen-Phen” (Hofmann rearr.)

RC

O

Cl

1) NaN3, heat

2) H2OR NH2

RC

O

NH2

NaOH, Br2

H2OR NH2

C O:NH3

C NH CH NH2

H2

Raneynickel

Reactions of Amines 1) Alkylation: Reaction of amines with alkyl halides results in substitution of alkyl

groups for the H atoms. Tertiary amines can be used to prepare quaternary ammonium salts in this way:

2) Amide formation: Reaction of carboxylic acids or acyl chlorides with amines 3) Hofmann eliminations: Alkenes from quaternary ammonium salts Quaternary ammonium salts can be prepared from amines using alkyl halides,

such as an excess of CH3I. NR3

+ becomes a leaving group In the presence of base ( – OH or Ag2O), β-elimination by E2 pathway occurs This reaction was more commonly used to identify amines than to make

alkenes Regiochemistry: When there is more than one set of β-hydrogens, elimination occurs from the less substituted carbon (“anti-Zaitsev” orientation). The tertiary amine is a bulky leaving group, so the less hindered position is favored.

:N(CH2CH3)3CH3CH2Cl

(CH3CH2)4N+ Cl-

N

CH3

CH3

H2C

CH2

H3C

H3C

heat H3CCH

CH2 + N

CH3

CH3

H3C

H3CC

Cl

O

H2NC

NH

OH2NCH3CH3

Mechanism of Hoffmann Rearrangement

Arylamine reactions Electrophilic aromatic substitution (Review): In general, amines are activators and direct substitution at the ortho & para positions Reactivity is enhanced so much that multiple substitutions often occur: Diazonium Salts, Azo compounds and Color Although electrophilic aromatic substitution is a versatile procedure, not all substituted aromatics can be made directly (fluorobenzenes, phenols) Transformation of aniline to give a good leaving group provides a route to a variety of substituents: the diazonium salt is such a group

NaNO2 and acid produce HNO2, which eliminates water to form nitrosonium ion (NO+)

Aryl diazonium salts can react with nucleophiles to release stable N2; this is the driving force behind many useful substitutions:

(Sandmeyer reaction)

(Schiemann rxn)

NH2

NaNO2 or HNO2

0oCN N

H2SO4

Cl-

N NHBr, CuBr

or HCl, CuClBror Cl

+ N2

N N

Cl-

HBF4

HeatF + N2 + BF3 + HCl

N N

Cl-

KCNC + N2CuCN

N

N N

Cl-

NaII + N2

N N

Cl-

Cu2OOH + N2

Cu(NO3)2,H2O

When you need an o, p director on a temporary basis: An amino group may be gotten rid of entirely by conversion to an azo group

followed by reaction with hypophosphorous acid:

Azo compounds…to dye for! The diazonium salt makes a “cool” electrophile (only stable at low temps) and is particularly effective in coupling reactions with activated benzenes such as phenols. Example: the ingrain dyeing process used to make colorfast azo dyes inside cotton fibers: Reaction between a phenol and a diazonium salt prepared from a substituted aniline

Azo dyes are molecules containing aromatic rings conjugated together by azo linkages and often having various substituents at different positions The extended conjugated pi system absorbs light in the visible range These pi systems, referred to as “chromophores,” are necessary for color The nitro, amino, chloro, bromo & hydroxy groups found as ring substituents

are called “auxochromes” and cause variations in the basic color of the dyes Phenols are particularly useful for dyeing cotton because their OH groups can

hydrogen-bond with cotton fibers (cellulose)

NH2

NaNO2 or HNO2

0oCN N

H2SO4

Cl-

H3PO2+ N2

Physiological roles of amines: Neurotransmitters and amines that mimic them

Amphetamines

CNS stimulants

Opiates (psychoactives)

Problems

How could these amines be prepared using reductive amination?

Predict the products that would form in a Hofmann elimination:

Compound A, C6H12O, has an IR absorption at 1715 cm-1 and gives compound B, C6H15N, when treated with NH3 and NaBH3CN. The IR and 1H NMR spectra of B are shown. What are the structures of A and B?

How would you prepare these compounds from benzene using a diazonium replacement? a. p-bromobenzoic acid b. m-bromochlorobenzene c. p-methylbenzoic acid

Amino Acids and Proteins Peptides & proteins = natural “polymers” made up of amino acid units (“residues”) Peptides are generally < 40 amino acids; proteins range in size from 40 aa's to several thousand Roles of proteins: Major structural component in animal kingdom:

--skin, bones, muscles, tendons contain mostly collagen --hair, fur, nails, feathers, hooves mostly keratin

Enzymes are primarily proteins; some hormones are proteins or peptides Structure is vitally important to function. There are several levels of protein structure: Primary structure: sequence of amino acids in the protein chain Secondary structure: folding or twisting of the protein backbone Tertiary structure: 3-D structure of the entire protein chain Quarternary structure: In proteins composed of more than one chain, how

these chains associate with each other The primary structure of a protein determines the other levels of structure. Thus, a good understanding of the units that make up the protein chain is important. Unlike polysaccharides, which are composed of the same type of unit linked in different ways, protein chains can contain any or all of the naturally-occurring amino acids Table 20.1: each amino acid makes up somewhere between 1.1 and 9.0 % of

the average protein Variety in amino acid structure leads to tremendous variety in protein aa

sequences These sequences determine protein properties & 3-D structure.

Amino acids: Bifunctional molecules General structure: where R = “side chains” of varying structure All a.a’s except glycine have a chiral center At physiological pH, both amino & acid groups are charged Stereochemistry: Unlike sugars, all amino acids in nature have the L-configuration defined by glyceraldehyde: COO- CHO CHO COO- H NH3 H OH HO H H3N H R + CH2OH CH2OH + R

D-amino acids D-glyceraldehyde L-glyceraldehyde L-amino acids All amino acid groups have the 1o amine and carboxylic acid functionality, which join together in amide linkages to make peptides and proteins. Side chain structure varies greatly: 1) Aliphatic amino acids: Contain alkyl side chains, all hydrocarbon, neutral &

nonpolar (“hydrophobic”) side chains: Gly, Ala, Val, Leu, Ile 2) Hydroxy-amino acids: Contain alcohol side chains, polar Ser, Thr 3) Sulfur-containing: Contain thiol or thioether side chains, less polar than OH Cys, Met 4) Acidic: Contains a second carboxylic acid group, very polar These side chains may be negatively charged at basic pH: Asp, Glu 5) Amides: Contain an amide of the acid groups noted above, also polar Asn, Gln 6) Basic: Contain a second amine group These side chains may be positively charged at acidic pH Arg, Lys, His 7) Aromatic: Contain a benzene ring Tyr (Trp) 8) Heterocyclic: Contain a heterocyclic amine ring Trp (His) Pro (side chain and amino group are together in a ring)

HC

NH3O

R

O

Acid-base properties of amino acids At pH = 7.4, both acid & amino groups are charged; typical amino acid is a zwitterion: Amino group Acid group Side chains pKa = 9 pKa = 2 pKa varies (table 26.1) At pH < 2, the acid group is protonated and so is the amino group. At pH > 2, the acid group loses its proton At pH > 9, the amino group loses its proton and becomes uncharged Under no condition does the molecule exist with both groups uncharged

Those amino acids with acidic or basic side groups may have an additional charge depending on the relationship between pKa and pH. In general: when pH < pKa, the group is protonated

when pH > pKa, the group loses the proton

Example: histidine, with a weakly basic side group has a pKa of 6.04

Amino acids can be identified by titration curves: illustrates pKa values of the groups Plot of pH vs. equivalents of added base pH at each inflection point on the curve corresponds to pKa of an ionizable

group titration curve for alanine shown

Isoelectric point (pI): Useful property in identification and separation of amino acids pI = pH at which a sample of amino acid has no net charge (the charges cancel) For aa’s with neutral side chains: pI = pKa (acid) + pKa (base) 2 For amino acids with ionizable side chains: Basic: pI = average of the Acidic: pI = average of the side chain pKa + amino group pKa side chain pKa + acid group pKa

How pI affects behavior: using charge to separate amino acids When solution pH < pI, protonation predominates: aa is positively charged overall When solution pH > pI, deprotonation predominates: aa is negatively charged overall The charge on an amino acid affects its response to an electric field at a given pH This property can be used to separate and identify amino acids in a mixture (electrophoresis) It can also be used preparatively in ion-exchange chromatography: Ion exchange resins

have charged side chains that either repel or attract amino acids depending on the buffer pH

TLC or paper chromatography can be used to separate amino acids by polarity Electrophoresis: An electric

field is applied to a buffered solution containing paper or gel

Amino acids are applied to the gel They migrate towards either positive or negative electrode depending on pI

If pI is below pH, aa will have (-) charge, migrates to positive If pI is above pH, aa will have (+) charge, migrates to negative

If pI values are close, the larger the molecule the slower it moves This principle can be applied to separation of peptides and proteins too, since

overall charge depends on charges of aa constituents as well as size Amino acids are detected by staining with ninhydrin, an imine-formation

reaction

Peptide bonds Peptides and proteins are composed of amino acids joined together by "peptide bonds": The reaction is essentially formation of an amide from a carboxylic acid and a 1o amine Ex: A tripeptide of valine, cysteine and serine: Val-Cys-Ser (valylcysteylserine) Val Cys Ser Direction & sequence matters: Val-Cys-Ser is different from Ser-Cys-Val Peptide bonds have partial double bond character No free rotation about the C - N bond The peptide group is planar α- carbons of adjacent aa are trans to each other Disulfide bonds are another type of covalent bonds found in peptides & proteins A mild oxidizing agent can link 2 thiols such as cysteine side chains by forming disulfide bridges which hold the cysteines together:

CHH3N

O

H2C

O

CHH3N

O

CH2

O

CHH3N

O

CH3

O

HS OH

CNH

O

H2C

O

C

HN

CH2

O

CH3N

H3C

OHS

OH

H

H H