Enant Chiral.presnt.

Copyright Steven Bottle, Chemistry, QUT, 1995 1

CHB 242 / 289 OCHB 242 / 289 ORGANICRGANIC

Organic chemistry of biologically important

functional groups containing O, N and S.

Lecture 3

Alkanes - Alkanes - Alkyl/Aryl HalidesAlkyl/Aryl Halides

General Formula: R-X X = F, Cl, Br, I R = alkyl or aryl group

Little importance in normal biological processes CHCl3 Early Anaesthetic CCl4 Dry Cleaning Fluid CF2Cl2 Freon Gas in

Fridges (Text: Pgs. 37 - 44)

Alkanes - Alkanes - Alkyl/Aryl HalidesAlkyl/Aryl Halides

Here is an example of a solvent commonly used by University students.

With trichloroethane this a solvent in liquid paper.

QuickTime™ and aGraphics decompressor

are needed to see this picture

Alkanes, Alkyl/Aryl Alkanes, Alkyl/Aryl HalidesHalides

Many halogenated compounds play a role in biological systems as insecticides.

ChlordaneDDT

NomenclatureNomenclature

Ordered alphabetically Numbered to give the lowest number

at the first point of difference:

2-chloro-2,3-dibromo-5-fluoro-3-iodo-4-methylpentane

CH2 C C C CH3

Nomenclature Nomenclature Alkyl Alkyl HalidesHalides

Ordered alphabetically Numbered to give the lowest number

at the first point of difference:

NOT: 4-chloro-2,3-dibromo-5-fluoro-3-iodo-2-methylpentane!

CH2 C C C CH3

Synthesis -Synthesis - Alkyl Halides Alkyl Halides

Addition to alkenes: Of Br2 for instance:

C C + Br2 C C

Synthesis -Synthesis - Alkyl Halides Alkyl Halides

Addition to alkenes: Or of HBr for instance:

C C + HBr C C

Synthesis -Synthesis - Aryl Halides Aryl Halides

Don’t forget we can make aryl halides via a substitution reaction:

catalyst

65%35%

++ Br Br

ReactionsReactions

Mostly involve displacement:

Halides are generally good leaving groups.

Br -+R O CC Br

Alkyl halides - ChiralityAlkyl halides - Chirality

Alkyl halides are not very significant biologically, but are useful to introduce the concept of chirality.

“Chiral” - from Greek meaning “hand”.

Look at your left and right hands Same gross physical characteristics Four fingers, one thumb, one palm etc.

(Text: Pgs. 250 - 258)

ChiralityChirality

The left and right hands have the same gross physical characteristics, but are different!

You cannot wear a left glove on your right hand.

The left and right hands are mirror images of each other.

Two molecules may also be mirror images of each other.

ChiralityChirality

Chirality arises in organic molecules as each group bound to a carbon occupies a specific place in space.

QuickTime™ and aGraphics decompressor

are needed to see this picture

ChiralityChirality

These two halomethanes are non-superimposable mirror images of each other and are said to be Chiral.

mirror

ChiralityChirality

These two halomethanes are non-superimposable mirror images of each other and are said to be Chiral.

ChiralityChirality The compounds are differentbecause A can not be super-imposed over B.Br Cl

ChiralityChirality

Two atoms align. But two don’t!

ChiralityChirality

Superimposing the Iodine and Fluorine atoms means the Chlorine and Bromine do not correspond (and vice versa).

ChiralityChirality

Not all mirror images are chiral (ie are non-superimposable).

mirror

ChiralityChirality

Not all mirror images are chiral (ie are non-superimposable).

ChiralityChirality

These two compounds are super-imposable mirror images.

ChiralityChirality

These two compounds are super-imposable mirror images.

ChiralityChirality

The other two are chiral isomers. H H

FI Cl Br

ChiralityChirality

Rule For Chirality: Four different groups attached to

carbon. H H

ChiralityChirality

Chiral compounds are special types of isomers.

They have the same molecular formulae and the same functional groups attached to them.

They differ only in the 3D arrangement of those groups.

ChiralityChirality

Non-superimposable mirror images are special isomers called ENANTIOMERS.

IBUPROFENHOOC

ChiralityChirality

It is important to realise that each enantiomer has exactly the same gross physical properties as its mirror image.

MB, BP, solubility, reactivity etc. are all the same.

However they interact with polarised light differently.

ChiralityChirality

Chiral substances are called optically active because they rotate the plane of polarised light.

Each enantiomer rotates the plane of polarised light in the opposite direction.

ChiralityChirality

(+)-2-iodobutane rotates the plane of light clockwise by 15.9°.

Its enantiomer (-)-2-iodomethane rotates the light anti-clockwise by 15.9°.

ChiralityChirality

(+)-2-iodobutane rotates the plane of light clockwise by 15.9°.

ChiralityChirality

Its enantiomer (-)-2-iodomethane rotates the light anti-clockwise by 15.9°.

ChiralityChirality

These compounds are enantiomers: non-superimposable mirror images.

ChiralityChirality

Rotate the molecule onthe right 180°.

ChiralityChirality

Iodine doesnot align.

ChiralityChirality

Iodine doesnot align.

ChiralityChirality If a substance rotates the light

clockwise (as you look at the source) it is called Dextrorotatory and the symbol (+) is placed in front of its name.

Enantiomers that rotate the light in the opposite direction are called Levorotatory and have the symbol (-).

ChiralityChirality

Importance of Chirality. The body (and indeed Nature) is filled

with chiral molecules, especially enzymes, which will only react with molecules of the same chirality.

Think of shaking hands. It must be done left to right or it won’t

ChiralityChirality - Examples - Examples O

(-) Carvone[]20 = -62.5 °D

(+) Carvone[]20 = +62.5 °D

(Spearmint oil) (Caraway oil)

ChiralityChirality - Examples - Examples

Minus sugar: The enantiomer of sucrose tastes just

as sweet, but it cannot be broken down by the body!

Unfortunately it must be made by chemists and not plants so it is expensive!

Sucrose and its mirror image.

Lindane - a common insecticide. This is the only activecompound!

One of the nine isomers produced

3Cl2light

Ibuprofen: Sold as a mixture of the two enantiomers.

Only one is biologically active!

Look at this compound. Can it be chiral?

Remember the rule for chirality is that a carbon must be bound to four different groups.

What would these isomers look like?

ChiralityChirality There is no real need to draw the

reflection to ascertain the structure of the two enantiomers. Chiral compounds exist because of the

differing positions of the four groups in space.

Switching two of the groups will produce the enantiomer for simple systems.

ChiralityChirality

Draw one enantiomer. NO O

ChiralityChirality

Then change the orientation of the groups.

ChiralityChirality

These two molecules are now non-superimposable.

ChiralityChirality

This only makes enantiomers for compounds containing one chiral carbon.

Remember enantiomers are non-superimposable mirror images.

Let us look at an example that demonstrates this.

ChiralityChirality

Let’s look at Peppermint:

This molecule can be chiral as there is a carbon which has four different groups attached.

ChiralityChirality

Let’s look at Peppermint:

This molecule can be chiral as there is a carbon which has four different groups attached.

This is indicated by the presence of a star (Asterisk).

ChiralityChirality

Here the four groups are in differing positions in space.

The molecules are not super imposable.

And they would bend polarised light. But they are NOT enantiomers!

ChiralityChirality

Enantiomers are mirror images. These two molecules are not

mirror images!

ChiralityChirality

Draw the mirror image of the molecule on the left and compare it.

ChiralityChirality

If you flip the reflected molecule you can attempt to superimpose the two.

ChiralityChirality

It appears that the two molecules on the right of this slide are the same .

ChiralityChirality

They are not the same because there is another chiral carbon in the molecule.

ChiralityChirality

The top methyl group is bound to a carbon that has four different groups.

Scrambling of the bonds to one of the chiral carbons does not produce a mirror image (ie an enantiomer).

ChiralityChirality

Take the molecule on the left and make its mirror image.

ChiralityChirality

Take the molecule on the left and make its mirror image.

ChiralityChirality

Flipping the reflection does not give a super-imposable compound!

ChiralityChirality

Flipping the reflection does not give a super-imposable compound!

The methyl groupsdo not line up.

ChiralityChirality

These are examples of yet another type of isomer!

They are optically active (ie chiral). But they are not mirror images. They are called diasteriomers.

ChiralityChirality

The situation is actually a little more complex as there is another chiral carbon in the molecule. Can you find it?

For ‘n’ chiral carbons in a molecule there are up to ‘2n’ optical isomers.

ChiralityChirality

Chirality - Chirality - RacematesRacemates

What happens when HBr is added to the alkene 1-butene?

An addition reaction occurs. The product is 2-bromobutane.

(The Markovnikov product) Is the product optically active?

The result is an addition which gives a chiral carbon.

C CH H

H CH2 CH3 HBr C C

H HH CH2 CH3

But in fact the addition can occur from either side!

Both compoundsare optically active,but the mixture is not.

H CH2 CH3 HBr C C

H HH CH2 CH3

* C C HH

CH2H CH3

A mixture that contains both enantiomers is called racemic.

Most chemical reactions produce racemic mixtures.

Most biochemical reactions produce only one optical isomer and are not racemic.

Chirality - Chirality - ReviewReview

Compounds can be chiral if there is a carbon which has four different groups.

Chiral compounds are said to be optically active as they can rotate the plane of polarised light.

Enantiomers are non super-imposable mirror images.

Chirality - Chirality - ReviewReview

Diasteriomers are optically active compounds which are not mirror images of each other.

Diasteriomers must have at least two chiral carbons.

Enant Chiral.presnt.

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