3.4 Alkenes
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What are some properties about alkenes?
Unsaturated hydrocarbons Homologous series Functional group of a double bond General formula CnH2n (only applies to one double bond) Insoluble in the water
Are alkenes more or less reactive than alkanes? Why?
More They have a double bond
Why are alkenes insoluble in water?
Water uses hydrogen bonding and alkenes use van der vaals Therefore they have incompatible intermolecular forces
What is the trend in boiling points relating to carbon chain length? Why?
As the carbon chain length increases the boiling point increases It has a larger surface area therefore stronger intermolecular forces are which are harder to overcome
What does the bonding in alkenes include?
Sigma bond Pi bond
What is stereoisomerism?
When molecules have the same molecular and structural formulae but a different arrangement of the atoms in space
Why does stereoisomerism occur in alkenes?
Because the double bond in alkenes does not allow rotation
What type of isomerism is stereoisomerism?
Geometrical isomerism or E-Z isomerism
In E-Z isomerism how do you determine which molecule is E or Z?
Z - the one with the larger atomic mass on one side E - the molecule with a smaller difference in masses on each side
What are the reactions of alkenes?
Combustion Electrophilic addition Electrophilic addition reaction with concentrated sulphuric acid Reaction with water
What are some points about combustion of alkenes?
They don’t burn very well often incomplete forming carbon Alkenes are made by cracking making them valuable and expensive for plastics
In electrophilic reactions what is the reagent?
Electrophile
What is an electrophile?
Attracted to negative charge (electrons) Therefore the less EN charged atom in the molecule (the positive delta one) acts as the electrophile
What are some examples of electrophiles?
HCl HBr HI Cl2 Br2 I2 H2SO4 H2O H2
What is an addition reaction?
Where 2 or more molecules join together by breaking the π bond to form a single product
What is the first part of the mechanism for electrophilic addition reactions?
Put the electrophile closest to the alkene in the vertical molecule An arrow from the double bond to the electrophile An arrow from the middle of the bond of the electrophile to the more EN atom attached (the delta negative one)
What is the second part of the mechanism for electrophilic addition reactions?
A temporary intermediate is formed called a carbocation An arrow goes from the free radical of the negatively charged atom to the carbon on the carbocation The carbon with only 3 bonds has a positive charge on it
What is the third part of the mechanism for electrophilic addition reactions?
Draw the (probably halogenoalkane) by putting the free radical in the gap on the carbon
What takes place in the first part of electrophilic addition reactions?
Heterolytic fission Meaning an atom takes both electrons as opposed to one to each
What is the carbocation scale?
Tertiary - secondary - primary
Explain the carbocations?
Tertiary - 3 carbons attached to the carbon with only 3 bonds Secondary - 2 carbons attached to the carbon with only 3 bonds Primary - 1 carbon attached to the carbon with only 3 bonds
What is important about the stability of carbocations?
When the alkene double bond breaks it has two possible outcomes Therefore you get more of the product with a more stable intermediate/carbocation
What is the stability of the carbocations?
The inductive effect of the alyl groups as they cancel out the positive charge The electrons are being pulled towards the positive charge
How can Br2 Cl2 etc… be used as electrophiles in electrophilic addition reactions?
Because of fluctuating electron density This causes a temporary induced dipole giving rise to Van der Waal forces The delta postive end will be attracted to the double bond making a permanent dipole as the double bonds electrons are available