+

AHL Organic ChemistrySyllabus

+20.1Prepare a student guide on naming of the following

6 C atoms MAXIMUM :- Amine Amide Ester Nitrile Alcohol Aldehyde Ketone Carboxylic acid Halide

MUST : Include functional group with 2 examples of each homologous series

SHOULD : include branched examples

COULD : include primary, secondary, tertiary

+Nucleophilic Substitution Reactions

Nucleophiles

Why is hydroxide better than water ?

+20.2.2 Identity of the halogen

Consider the R-X bond

RCl < RBr < RI

+20.2.3 Nature of halogen effect on rate

Review : Sn1 Sn2 reaction mechanisms, and rate data

Rate : Tertiary > Secondary > Primary

Sn1 route is faster as transition state ofSn2 has a higher activation energy.

+Study Questions

Why don’t primary halogenoalkanes use the Sn1 mechanism.

Why don’t tertairy halogenoalkanes use Sn2 ?

+Starter

Explain how you would synthesise 2-methyl-amino-propane from the appropriate haloalkane.

+Draw structural formulae of …..

1,2-dichlorobenzene Describe the bonding in this structure

3-chlorohexanamide Describe the intermolecular forces present in this molecule

The first 4 organic acids Describe and explain the boiling point trend.

+Recap – what affects the relative rate of Sn1 Sn2 reactions Nature of leaving group

Primary >> Secondary >> Tertiary

For Sn2 increasing nucleophile concentration increases rate – no effect on Sn1.

Increasing nucleophile strength favours Sn2 – hydroxide >>> water

+Completing the picture

Substitution happens in competition with elimination reactions.

Both reactions require REFLUX with NaOH ( or KOH )

What decides elimination or substitution ? Primary – mainly substitution Tertiary – mainly elimination Water solvent favours substitution Ethanol solvent favours elimination Higher concentrations ( at elevated temperature ) of alkali

favour elimination

+E2 mechanism ( E2 not on IB but..)

Elimination from symmetrical haloalkanes.

Elimination from unsymmetrical haloalkanes.

+Stereoisomerism

What ? compounds with the same structural formula but with

different arrangements of atoms in space.

So give me some examples …… Consider 1,2-dibromoethane and 1,2-dibromoethene.

+Cyclics also exhibit geometric isomerism

Cyclopropane

Cyclobutane

+Physico-chemical properties of geometric isomers

Cis and trans 1,2-dichloroethene boiling points are 60C and 47C – why ?

Cis and trans but-2-ene-1,4-dioic acid Cis forms the anydride BUT trans has no reaction……

Constitutional Isomers

Have same molecular form but different structural formula

Both isomers to the right are C4H10

These have different chemical and physical properties

H3C

CH2

CH2

CH3

H3CCH

CH3

CH3

butane

isobutane

B.P = -0.5C

B.P = -11.7 C

Stereoisomers

Appear to have same structure in 2D

In 3D have different structures

Are non-superimposable mirror images of each other

With modeling kit build a model: carbon atom with four different colors attached (use same size bonds!)

Look at your model in a mirror, build what you see in the mirror.

Are the two superimposable?

All molecules have a mirror image – but for most molecules it is the same molecule.

fluoromethane

H

CH F

H

H

CHF

H

+

For some molecules the mirror image is a different molecule (the mirror image is non-superimposable).

OH

CH CH3

COOH

OH

CHH3C

HOOC

(-) lactic acid (+) lactic acidin sour milk in muscles

• Left and right hands are an example of non-superimposable mirror images.

Chirality Game on blog please

This usually happens when a molecule contains a C atom with four different groups attached (chiral / asymmetric C).

Such molecules are said to be chiral or optically active.

a

Cb c

d

a

Cbc

d

The optical isomers are called enantiomers.

These are distinguished by +/-, D/L or more correctly R/S.

A 50/50 mixture of the two enantiomers is called a racemic mixture or a racemate.

TASK Which of the following molecules are optically active?

1) propan-2-ol2) 2-chlorobutane3) 1-chlorobutane4) 3-methylhexane

5) butanone6) 2-methylbutanoic acid7) butan-2-ol8) 1-chloro-3-methylpentane

propan-2-ol

NOT OPTICALLY ACTIVE

CH3 CH CH3

OH

2-chlorobutane CH3 CH CH2 CH3

Cl

OPTICALLY ACTIVE

CH2CH3

CH Cl

CH3

CH2CH3

CHCl

H3C

1-chlorobutane

NOT OPTICALLY ACTIVE

CH2 CH2 CH2 CH3

Cl

3-methylhexane

OPTICALLY ACTIVE

CH2 CH CH2 CH2

CH3

CH3CH3

CH2CH2CH3

CH CH2CH3

CH3

CH2CH2CH3

CHCH3CH2

CH3

butanone

NOT OPTICALLY ACTIVE

C CH2 CH3CH3

O

2-methylbutanoic acid

OPTICALLY ACTIVE

CH3 CH2 CH

CH3

C

O

OH

CH2CH3

CCH3 COOH

H

CH2CH3

CCH3

HHOOC

butan-2-ol

OPTICALLY ACTIVE

CH3 CH2 CH

OH

CH3

CH2CH3

CCH3 OH

H

CH2CH3

CCH3

HHO

1-chloro-3-methylpentane

OPTICALLY ACTIVE

CH3 CH2 CH

CH3

CH2 CH2

Cl

CH2CH3

CCH3 CH2CH2Cl

H

CH2CH3

CCH3

HCH2ClCH2

How to draw 3D: Dash-Wedge Formula Draw two straight lines

about 110 from each other Bond angle is 109.5 These represent the bonds

in the plane of the page

Draw dashed line for the bond that extends behind the plane of the page

Draw wedge for bond that extends in front of the plane of the page

OH

H

Cl

CH3

+Drawing Enantiomers

Draw the molecule you built, using the dash-wedge formula and colored pens

Use a mirror to see the enantiomer

Sketch the mirror image, using the dash-wedge formula

Draw the two stereoisomers of 1-chloro-1-bromo-ethane. There is no convention for which atom is attached to a

wedge.

Chiral centers

Carbon attached to 4 different substituent groups Even if two substituent groups start with C,

they could still be different

C=O, never chiral (C only attached to 3 things)

CH3, CH2 groups, never chiral because they are symmetric:

OH

H

CH2CH3

H3C

OH

H

H3CH2C

CH3

+Vocabulary

Stereoisomers have chiral centers.

Carbon atoms can be chiral or achiral Asymmetric or symmetric

If two molecules are stereoisomers, they are also called enantiomers

Chiral molecules are optically active

Enantiomers are optical isomers

+Properties of Optical Isomers

same physical and chemical properties

rotate plane polarized light Each isomer rotates it in a different direction What is plane polarized light?

Try polarizers

+Optical Activity

Stereoisomers are said to be optically active if the rotate plane polarized light

Each type of enantiomer rotates light the same amount, but in different directions.

Amount and direction of rotation must be experimentally determined using a polarimeter

Optical Activity

The amount of rotation depends on Length of sample tube Concentration of enantiomers

Both isomers present, rotation cancels out called: racemic mixture

Molecules that are optical isomers are called enantiomers.

• Enantiomers have identical chemical and physical properties, except:• Their effect on plane polarised light;• Their reaction with other chiral

molecules

Light is a form of electromagnetic radiation.

normal light(waves vibrate in all directions)

plane-polarised light(vibrates in only one direction)

plane-polarised light after clockwise rotation

The wave vibrations are perpendicular to the direction of travel of the wave.

Optical isomers rotate the plane of plane polarised light.

(-)-enantiomer(anticlockwise rotation)

(±)-racemate(no overall effect)

(+)-enantiomer(clockwise rotation)

+

• Chiral molecules often react differently with other chiral molecules.

• This is like the idea that a right hand does not fit a left handed glove – the molecule must be the correct shape to fit the molecule it is reacting with.

• Many natural molecules are chiral and most natural reactions are affected by optical isomerism.

• For example, most amino acids (and so proteins) are chiral, along with many other molecules.

• In nature, only one optical isomer occurs (e.g. all natural amino acids are rotate polarised light to the left).

Many drugs are optically active, with one enantiomer only having the beneficial effect.

In the case of some drugs, the other enantiomer can even be harmful, e.g. thalidomide.

In the 1960’s thalidomide was given to pregnant women to reduce the effects of morning sickness.

This led to many disabilities in babies and early deaths in many cases.

+

H2CCH2

C

NHO O

H

N

O

O

H2CCH2

C

NH OO

H

N

O

O

S thalidomide (effective drug)

The body racemises each enantiomer, so even pure S is dangerous as it converts to R

in the body.

R thalidomide (dangerous drug)

Thalidomide was banned worldwide when the effects were discovered.

However, it is starting to be used again to treat leprosy and HIV.

Its use is restricted though and patients have to have a pregnancy test first (women!) and use two forms of contraception (if sexually active).

S carvone (caraway seed) R carvone (spearmint)

O

CH3

H C CH2

H3C

O

CH3

HCH2C

CH3

Caraway Seed has a warm, pungent, slightly bitter flavour with aniseed

overtones.

S limonene (lemons) R limonene (oranges)

CH3

HCCH2

CH3

CH3

H C CH2

H3C