Download Alkenes: Overview

Alkenes: Overview
•These compounds involve a carbon - carbon DOUBLE bond.
•This double bond is the source of the reactivity of these molecules.
•One of the major types of reactivity is ADDITION.
•The colour of Br2 disappears!
This can be used to detect the presence of a C - C double bond.
•Addition of halogens such as Br2 is only one example of such addition reactions.
•Hydrogen (H2), Water (H2O), Acids (HCl, HBr, etc)
Alkenes : cis / trans isomerism
•There are isomers that exist because unlike single bonds the C - C double bond cannot
rotate.
•Restricted Rotation and the Double Bond
•There is a large energy barrier to rotation (about 264 kJ/mol) around the double bond
•The rotational barrier of a carbon-carbon single bond is 13-26 kJ/mol
•Cis-trans isomers are the result of restricted rotation about double bonds
Cis-trans isomerism is not possible if one carbon of the double bond has two identical
groups
Alkenes: Conjugation: Sequential double bond/single bonds have increased stability.
•This is 1,3-cyclohexadiene.
CH
H2C
CH
H2C
CH
CH
•What happens when there are three double bonds within a six-membered ring?
The compound is called benzene.
•Benzene is represented by the following symbol:
It has the formula C6H6 which can be represented two ways
•Are the two forms different? NO!
•The structure should be thought of as existing somewhere between these two forms:
•We write this using a circle within the ring of six carbons:
•Benzene is not an ‘average’ of two structures, it exists as a symmetrical system with
each carbon equivalent to each other carbon.
Benzene
•In 1825 London’s new gas street lamps had a problem.
•Liquid (solid if < 6° C) was condensing in the gas lines and blocking the flow.
•Michael Faraday isolated the unknown compound and tried to characterise it.
•Combustion gave C6H6 - Benzene.
•Put yourself in Faraday’s place.
•C6H6 suggests unsaturation. (Alkanes are normally CnH2n+2)
•Logically the unknown compound should decolourise Br2.
•So what happened? Nothing!
•Benzene and other aromatic hydrocarbons do not undergo addition reactions.
•In this regard they should not be thought of as alkenes.
•Benzene can be made to react with Br2, but only in the presence of a catalyst.
•And the reaction is a substitution and not an addition!
Benzene: Substitution Reactions.
Benzene
•Benzene is only one example of a class of compounds called aromatics.
•They are all stabilised by aromaticity.
•The term originally referred to their pleasant smell.
•Non-aromatics were called aliphatic.
•Phenols are special examples of alcohols.
•Molecules where the hydroxyl group is bound to an aromatic ring are phenols.
•Phenols display similar hydrogen bonding properties as alcohols.
•The OH bond is polarised, solubility and boiling point are affected.
Phenols
•Substitution around the ring is indicated by ortho, meta or para.
The simplest phenol is phenol itself.
OH
Numbers may also be used:
4-bromo-3-methylphenol
•There are many examples of phenols:
CH3
•Tetrahydrourushiol - Poison Ivy
Br
•Thymol - Hookworms
•Phenols differ from alcohols as they are slightly acidic.
•A solution of Phenol (ie Phenol itself) in water is known as Carbolic Acid.
•This was used 130 years ago by Lister to kill bacteria.
•Lister’s use of this antiseptic revolutionised surgery.
•Phenol itself is too acidic. It burns flesh.
•Note the similarity between common mouth wash and the irritant from poison ivy:
•These two phenols yet again demonstrate that the structure and shape of molecules
governs their biological properties.