Topic 20 - Organic chemistry

• Introduction- functional groups and naming• Nucleophilic substitution reactions• Elimination reactions• Condensation reactions• Reaction pathways• Stereoisomerism

20.1 Ester• Functional group: -COOC- • Condensation reaction or esterification• Many fruit-smelling esters• The bond in triglycerides (lipids)

• Alcohol + carboxylic acid ester + water• CH3-OH + HOOC-CH3 CH3OOC-CH3 + H2O

(The underlined forms water)

Esterification

Name: – The alcohol part: Stem + yl: Methyl-– The acid part: the salt name; -buthanoate

=> Methyl-buthanoate

Methanol

Butanoic acid

Which names?

Which names?

Ethylethanoate propylethanoate

2-butylethanoate penthylpropanoate

Amine

• Relatives to ammonia => weak bases• Functional group –NH2

• H-bonds => higher bp’s, smaller ones are water-soluble etc.

• Name: stem + suffix: -ylamine (or prefix amino-)

– Methylamine CH3-NH2

– Ethylamine CH3-CH2-NH2

Amide

• Functional group: -CONH• Name: stem + suffix: -anamide

– Methanamid H-CONH2

– Ethanamid CH3-CONH2

• Peptide bond in proteins

Nitrile

• Functional group: -CN• Former name: cyanides

• Name: stem + suffix: nitrile– Metanenitrile: HCN– Ethanenitrile: CH3-CN

Nucleophiles and electrophiles- often needed in organic reactions

• Nucleophile- nucleus lover

• Has free electronpair and whole or part negative charge

• The larger the negative charge - the better the nucleophile

• Eg: C=C, H2O, -OH, -CN, NH3

• Electrophile-electron lover

• Has whole or part positive charge

• The larger the positive charge - the better the Electrophile

• Eg: C=O, H+, C-Cl,

20.2 Nucleophilic substitution reactions

Nu: + R-X Nu-R + X:

It’s important to know the difference of these because they will undergo different forms of nucleophilic substitution reactions

C Cl

H

H

C

H

H

H

C

H

Cl

C

H

H

H

C

H

H

H

C

CH3

Cl

C

H

H

H

C

H

H

H

Primary C with Cl and 2 H

Secondary C with Cl and 1 H

Tertiary C with Cl and 0 H

SN2 or SN1?SN2

• Bimolecular = two species in the rate determining step. Rate = k [org]*[Nu]

• Primary halogenoalkanes• Steric hindrance• One-step with transition

state• Inversion of configuration

SN1

• Monomolecular = one species in the rate determining step. Rate = k [org]

• Tertiary halogenalkanes• Heterolytic fission of substrate

rate determining step• Formation of inermediate

carbocation• Racemix mixture formed

Factors affecting the rate

• The halogen in the halogenoalkane is important for the reaction rate:

C-I > C-Br > C-Cl > C-F (cf. bond strength)• The stronger the nucleophile, the faster the

SN2-reaction

CN- > OH- > NH3 > H2O

SN2 substitution reaction with ammonia

H3N: + R-X R-NH2 + HX

Forming an amine

SN2 substitution reaction with CN- as nucleophile

R-X + CN- R-CN + X- Forming a nitrile

The carbon chain will be prolonged with one carbon atom

Reactions with nitriles

• Nitrile + H2 Ni catalyst Amine

CH3CH2-CN + H2/Ni CH3CH2-NH2

• Nitrile + Acidic water Carboxylic acid CH3CH2-CN + H+/H2O CH3CH2-COOH

20.3 Elimination reactions

• CH3CH2-Br + OH- CH2=CH2 + H2O + Br-

Condition: Hot and concentrated sodium hydroxide solution in ethanol. Reflux.

• Opposite to addition reaction• 2 kinds; E1 and E2

• If the conditions are different: A substitution reactionCH3CH2-Br + OH- CH3CH2-OH + Br-

Will happen in warm dilute solution of sodium hydroxide, the hydroxide ion will work as a nucleophile.

The E1 mechanism (similar to SN1)

• Involves heterolytic fission of C-X bond• Tertiary halogenalkane• Via a carbocation intermediate

The E2 mechanism (similar to SN2)

• Concerted bimolecular reaction• Primary halogenalkane• Via transition state

Dehydration reaction of alcohol to form alkene

• E1 or E2?• Excess of H2SO4, ~170oC

20.4 Condensation reaction• 2 Organic molecules 1 Organic molecule + water

• Acid catalysed• Esterification, etherification• Common in biochemistry, formation of polysaccharides,

proteins, nucleic acids

Esterification

Name: R-yl R’-oateEsters: flavouring agents (food, perfume), solvents, plastics (polyesters)

C OH

H

H

C

H

H

H

COH

O

C

H

H

H

C

H

H

C

H

H

H

C

O

C

H

H

H

OH+

+

Ethanol Etanoic acid Etylethanoate Water

H2O

C NH2

H

H

C

H

H

H

COH

O

C

H

H

H

C

H

H

C

H

H

H

C

O

C

H

H

H

N

H

+ H2O

Amide condensation

Amine Carboxylic acid Amide Water

H2N CH

CH3

COH

O

HN CH

H

COOH H2N CH

CH3

O

N CH2 COOH

H

Alanine Glycine

H

C +

Dipeptide of alanine and glycine

H2O

PolycondensationsPlastics

• Polyesters: polyethylene terphtalate (PET)

• Polyamides; nylone

Benzene-1,4-dicarboxylic acid + Ethane-1,2-diol

Can condense with a diolCan condense with a dicarboxylic acid

Monomers with two functional groups are required

Polyamides

HOOC-R-COOH + H2N-R’-NH2 H2N-R’-NH-OC-R-COOH Di-amine amide bond

HOOC-(CH2)4-COOH + H2N-(CH2)6-NH2

HOOC-(CH2)4-CONH-(CH2)6-NH2 + water

hexanedioic acid + 1,6-diaminohexane Nylon

Reaction pathways

1. Elimination reaction. Hot, concentrated and reflux2. Substitution reaction. SN1 or SN2

3. Substitution reaction. SN1 or SN2. (Can be substituted up 4 times to a quarternary ammonum salt)

4. Condensation reaction. Acid catalyst (or alkaline catalyst, but more common when hydrolysis of ester). Equilibrium reaction.

5. Condensation reaction. Acid catalyst (or alkaline catalyst). Equilibrium reaction.

6. Nitrile to amin: Reduction with H2 and Ni-catalyst

Stereoisomerism

• Different location in space of atoms or groups

Structural isomers

• Chain: e.g. n-butane : methylpropane

• Positional: e.g. 1-propanol : 2-propanol

• Functional groups: Ethanoic acid, CH3-COOH : Methyl methanoate HCOOCH3

Propanal, CH3-CH2-CHO : Propanone, CH3-CO-CH3

Geometric

• Double bond = p and s bonds. No free rotation around a double

cis-2-buten trans-2-buten

• cis-1,2-dichloroethane: bp = 60.3oC• trans-1,2-dichloroethane: b.p. = 47.5oC

cis-but-2-ene-1,4.dioic trans-but-2-ene-1,4.dioic

m.p = 286oC m.p.= 130oC with decomposition.• Strong H-bond between molecules in trans. Strong H-bond in the

molecule in cis.

Cyclic compoundscan also give geometric isomers

• Cis and trans

Optical

mirror plane

enantiomers to each other

Stereo isomers Different location in space.

Geometric

cis-2-buten trans-2-buten

Optical

mirror plane enantiomers to each other

• A carbon with 4 different substituents is said to have an asymmetric centre or to be chiral.

• Chiral can also mean the whole molecule.

• Enantiomers have the same physical properties except that they rotate the plane of polarised light in different directions.

• Very important which form of the enantiomer in biology and medicine.

Polarimeter

• Light: Electromagnetic radiation. Normal light oscillating in all directions.

• Plane-polarized light: When normal light is sends throuhg a polarizing filter only waves in the same plan can pass. If two polarizing filters places 90o to each other the light will be compleataly blocked.

• Polarimeter: Measure how much the light will rotate when

pass through a solution with optically avtive substance. A sample holder between two filter