specification from ocr oexplain the basicity of amines in terms of proton acceptance by the nitrogen...

Specification from OCR

o Explain the basicity of amines in terms of proton acceptance by the nitrogen lone pair.

o Describe the reactions of amines with acids to form saltso Describe the preparation of:

i) aliphatic amines by substitution of halogenalkanes with excess ethanolic ammoniaii) aromatic amines by reduction of nitroarenes using tin and conc. hydrochloric acid

o Describe the synthesis of an azo-dyeo State the use of reactions in the formation of dyestuffs

AminesAmines

Amines are essentially molecules of ammoniaAmines are essentially molecules of ammonia One or more of the hydrogen atoms have been One or more of the hydrogen atoms have been

replaced with an alkyl group.replaced with an alkyl group.

HN

H H

STRUCTURE & CLASSIFICATIONSTRUCTURE & CLASSIFICATION

Structure Contain the NH2 group

Classification

primary (1°) amines secondary (2°) amines

tertiary (3°) amines quarternary (4°) ammonium salts

Aliphatic methylamine, ethylamine, dimethylamine

Aromatic NH2 group is attached directly to the benzene ring (phenylamine)

R N:

H

H

R N:

R

H

R N:

R

R

R

+R N R

R

NOMENCLATURENOMENCLATURE

NomenclatureNamed after the groups surrounding the nitrogen + amine

C2H5NH2 ethylamine

(CH3)2NH dimethylamine

(CH3)3N trimethylamine

C6H5NH2 phenylamine (aniline)

PHYSICAL PROPERTIESPHYSICAL PROPERTIES

Boiling point Boiling points increase with molecular mass

Amines have higher boilingpoints than correspondingalkanes because of theirintermolecular hydrogen bonding

Quarternary ammonium

salts are ionic and exist as salts

Solubility Lower mass compounds aresoluble in water due to hydrogenbonding with the solvent.

Solubility decreases as themolecules get heavier.

Soluble in organic solvents.

PHYSICAL PROPERTIESPHYSICAL PROPERTIES

The LONE PAIR on the nitrogen atom in 1°, 2° and 3° amines makes them ...

BASES - they can be proton acceptors

RNH2 + H+ ——> RNH3+

NUCLEOPHILES - provide a lone pair to attack an electron deficient centre

Amines as basesAmines as bases

Bases are proton acceptors. Bases are proton acceptors. Amines don’t actually accept protons, they Amines don’t actually accept protons, they

donate a lone pair to the hydrogen atom to donate a lone pair to the hydrogen atom to form a dative bond. form a dative bond.

Ammonia and bases can do this with any Ammonia and bases can do this with any suitable acid to give a salt.suitable acid to give a salt.

H3N H Cl NH4+Cl-

BASIC PROPERTIESBASIC PROPERTIES

Bases The lone pair on the nitrogen atom makes amines basic;

RNH2 + H+ ——> RNH3+ a proton acceptor

Strength depends on the availability of the lone pair and its ability to pick up

protons

• the greater the electron density on the N, the better it can pick up

protons

• this is affected by the groups attached to the nitrogen





protons


protons


electron withdrawing substituents (benzene rings)decrease basicity as the electron density on N islowered and the lone pair is less effective

C6H5 N:

H

H





protons


protons


electron withdrawing substituents (benzene rings)decrease basicity as the electron density on N islowered and the lone pair is less effective

electron releasing substituents (CH3 groups)increase basicity as the electron density isincreased and the lone pair is more effective

CH3 N:

H

H

C6H5 N:

H

H


The strength of a weak base will depend on whether the substituents are electron-relasing or withdrawing

Compound Formula Comments

ammonia NH3

methylamine CH3NH2 methyl group is electron releasing

phenylamine C6H5NH2 electrons delocalised into the ring

strongest base > > weakest base

CHEMICAL REACTIONS - CHEMICAL REACTIONS - WEAK BASESWEAK BASES

Water Amines which dissolve in water produce weak alkaline solutions

CH3NH2(g) + H2O(l) CH3NH3+(aq) + OH¯(aq)

Acids Amines react with acids to produce salts.

C6H5NH2(l) + HCl(aq) ——> C6H5NH3+Cl¯(aq) phenylammonium

chloride

This reaction allows one to dissolve an amine in water as its salt.

Addition of aqueous sodium hydroxide liberates the free base from its salt

C6H5NH3+Cl¯(aq) + NaOH(aq) ——> C6H5NH2(l) + NaCl(aq) +

H2O(l)

The preparation of amines

1. PREPARATION OF ALIPHATIC AMINES1. PREPARATION OF ALIPHATIC AMINES

Aliphatic amines can be prepared from substitution of halogenoalkanes with substitution of halogenoalkanes with excess ethanolic ammoniaexcess ethanolic ammonia

Reagent Excess alcoholic ammonia

Conditions Reflux in aqueous, alcoholic solution under pressure

Product Amine

Nucleophile Ammonia (NH3)

Equation C2H5Br + NH3 (aq / alc) ——> C2H5NH2 + HBr

WHY IS THE AMMONIA IN EXCESS??

Reagent Aqueous, alcoholic ammonia (in EXCESS)Conditions Reflux in aqueous , alcoholic solution under pressureProduct AmineNucleophile Ammonia (NH3)

Equation e.g. C2H5Br + 2NH3 (aq / alc) ——> C2H5NH2 + NH4Br

(i) C2H5Br + NH3 (aq / alc) ——> C2H5NH2 + HBr

(ii) HBr + NH3 (aq / alc) ——> NH4Br

Mechanism

Notes The equation shows two ammonia molecules.The second ammonia molecule ensures the removal of HBr,

because otherwise the salt ethylammonium bromide C2H5NH3+Br¯ will be formed.

NUCLEOPHILIC SUBSTITUTIONNUCLEOPHILIC SUBSTITUTION

WHY IS THE AMMONIA IN EXCESS??

Why excess ammonia?A large excess ammonia also ensures that further substitution doesn’t take place - see below

ProblemAmines are also nucleophiles (lone pair on N) and can attack another molecule of halogenoalkane to produce a 2° amine. This too is a nucleophile and can react further producing a 3° amine and, eventually an ionic quarternary ammonium salt.

C2H5NH2 + C2H5Br ——> HBr + (C2H5)2NH diethylamine, a 2° amine

(C2H5)2NH + C2H5Br ——> HBr + (C2H5)3N triethylamine, a 3° amine

(C2H5)3N + C2H5Br ——> (C2H5)4N+ Br¯ tetraethylammonium bromide

a quaternary (4°) salt

2. Reduction of nitrobenzene to 2. Reduction of nitrobenzene to give phenylaminegive phenylamine

NO2

+ 6[H]i) conc. HCl/Sn

ii) NaOH(aq)

NH2

+ H2O

Conditions are reflux, this is important in the production of compounds called azo-dyes.

AMINO ACIDSAMINO ACIDS

Structure Amino acids contain 2 functional groups

amine NH2

carboxyl COOH

They all have a similar structure - the identity of R varies

H2N C COOH

H

H

H2N C COOH

CH3

H

H2N C COOH

R

H

AMINO ACIDS – AMINO ACIDS – OPTICAL ISOMERISMOPTICAL ISOMERISM

Amino acids can exist as optical isomersIf they have different R1 and R2 groups

Optical isomers exist when a moleculeContains an asymmetric carbon atom

Asymmetric carbon atoms have fourdifferent atoms or groups attached

Two isomers are formed - one rotates planepolarised light to the left, one rotates it to the right

Glycine doesn’t exhibit optical isomerism asthere are two H attached to the C atom

H2N C COOH

CH3

H

H2N C COOH

H

H

GLYCINE2-aminoethanoic acid

AMINO ACIDS - AMINO ACIDS - ZWITTERIONSZWITTERIONS

Zwitterion • a dipolar ion

• has a plus and a minus charge in its structure

• amino acids exist as zwitterions

• give increased inter-molecular forces

• melting and boiling points are higher

H3N+ C COO¯

R2

R1

• amino acids possess acidic and basic properties

• this is due to the two functional groups

• COOH gives acidic properties

• NH2 gives basic properties

• they form salts when treated with acids or alkalis.

H2N C COOH

R2

R1

AMINO ACIDS - AMINO ACIDS - ACID-BASE PROPERTIESACID-BASE PROPERTIES

AMINO ACIDS - AMINO ACIDS - ACID-BASE PROPERTIESACID-BASE PROPERTIES

Basic properties:

with H+ HOOCCH2NH2 + H+ ——> HOOCCH2NH3+

with HCl HOOCCH2NH2 + HCl ——> HOOCCH2NH3+ Cl¯

Acidic properties:

with OH¯ HOOCCH2NH2 + OH¯ ——> ¯OOCCH2NH2 + H2O

with NaOH HOOCCH2NH2 + NaOH ——> Na+ ¯OOCCH2NH2 + H2O

PEPTIDES - PEPTIDES - FORMATION & STRUCTUREFORMATION & STRUCTURE

Amino acids can join together to form peptides via an amide or peptide link

2 amino acids joined dipeptide

3 amino acids joined tripeptide

many amino acids joined polypeptide

a dipeptide

PEPTIDES - PEPTIDES - HYDROLYSISHYDROLYSIS

Peptides are broken down into their constituent amino acids by hydrolysis

• attack takes place at the slightly positive C of the C=O

• the C-N bond is broken

• hydrolysis with water is very slow

• hydrolysis in alkaline/acid conditions is quicker

• hydrolysis in acid/alkaline conditions (e.g. NaOH) will produce salts

with HCl NH2 becomes NH3+Cl¯

H+ NH2 becomes NH3+

NaOH COOH becomes COO¯ Na+

OH¯ COOH becomes COO¯



H2N C CO

CH3

H

NH C CO

H

H

NH C COOH

CH3

CH3

Which amino acids are formed?



H2N C CO

CH3

H

NH C CO

H

H

NH C COOH

CH3

CH3

CH3

H

H2N C COOH

H

H

H2N C COOH

CH3

CH3

H2N C COOH++

H2N C CO

CH3

H

NH C CO

H

H

NH C COOH

H

CH3



Which amino acids are formed?

H2N C CO

CH3

H

NH C CO

H

H

NH C COOH

H

CH3



CH3

H

H2N C COOH

H

H

H2N C COOH2 x +

PROTEINSPROTEINS

• are polypeptides with high molecular masses • chains can be lined up with each other • the C=O and N-H bonds are polar due to a difference in electronegativity • hydrogen bonding exists between chains

dotted lines ---------- represent hydrogen bonding

AMIDESAMIDES

Structure derivatives of carboxylic acids

amide group is -CONH2

NomenclatureWhite crystalline solids named from the corresponding acid(remove oic acid, add amide)

CH3CONH2 ethanamide (acetamide)

C2H5CONHC6H5 N - phenyl propanamide - the N tells you the

substituent is on

the nitrogen

Nylons are examples of polyamides

Preparation Acyl chloride + ammonia

CH3COCl + NH3 ——> CH3CONH2 + HCl ethanoyl chloride ethanamide

AMIDES - AMIDES - CHEMICAL PROPERTIESCHEMICAL PROPERTIES

Hydrolysisgeneral reaction CH3CONH2 + H2O ——> CH3COOH + NH3

acidic soln. CH3CONH2 + H2O + HCl ——> CH3COOH + NH4Cl

alkaline soln. CH3CONH2 + NaOH ——> CH3COONa + NH3

Identification Warming an amide with dilute sodium hydroxide solution andtesting for the evolution of ammonia using moist red litmus

paperis used as a simple test for amides.

ReductionReduced to primary amines: CH3CONH2 + 4[H] ——> CH3CH2NH2 + H2O

specification from ocr oexplain the basicity of amines in terms of proton acceptance by the nitrogen...

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