14.2 Reactions of alkenes Flashcards
Do alkenes combust in air?
Yes, they do to make carbon dioxide and water
However they are not used as fuels as they are reactive which makes them very useful for other purposes
Describe electrophilic addition reactions
The four electrons in the carbon - carbon double bond make it a centre of high electron density
So electrophiles are attracted to it and can form a bond by using two of the four electrons in the carbon-carbon double bond
What are electrophiles
These can accept a pair of electrons
So they are generally electron poor so are positively charged ions or have a positively charged area
Describe mechanism of electrophilic addition
. Electrophile is attracted to the double bond
. Electrophiles are positively charged so will accept a pair of electrons from the double bond
. A positive ion ( a carbocation is formed)
. A negatively charged ion will form a bond with this carbocation
Describe the electrophilic addition mechanism of ethene with a hydrogen halide eg HBr
. Bromine is more electronegative than hydrogen, so the HBr molecule is polar, and H is f+ and Br is f-
. The electrophile is the H f+ part of the molecule as all the electrons are attracted to electronegative bromine
. The H f+ part is attracted to C=C bond because of the double bonds high electron density
. One of the pairs of electrons from the double bond forms a bond with the H f+ part, to form a carbocation that is a positive ion
. Whilst meanwhile, the electrons in the HBr bond are drawn towards the Br-
. The bond in the hydrogen bromine breaks heterolytically
. Both electrons from the shared pair go to the bromine atom because it is more electronegative than hydrogen
. So the hydrogen joins to the organic molecule and the :Br- ion is alone
. However, because there is a C on the carbocation that is positively charged, the bromine ion will form a bond with it using its electron pair
What are asymmetrical alkenes
Eg in electrophilic addition with bromine
When the double bond in the alkene is not exactly in the middle of the chain, there are two possible products
As the bromine can add to either one of those carbons of the double bond
Eg propene could make 1-bromopropane or 2-bromopropane
However the product of it is almost entirely 2-bromopropane
Explain the reasoning behind why the product is almost always
2-bromopropane instead of
1-bromopropane
(positive inductive effect)
Alkyl groups eg -CH3 or -C2H5 have a tendency to release electrons
This is positive inductive effect, and is sometimes represented by an arrow along their bonds to show direction of the release
The electron releasing effect tends to stabilise the positive charge of the intermediate carbocation
So the more alkyl groups there are attached to the positively charged carbon atom, the more stable it is
So a tertiary carbocation (that is binded to 3 other alkyl groups) is more stable than a primary carbocation only binded to one.
Eg a carbon at the end of a molecule, binded to 3 hydrogens and one carbon
Describe the reaction of alkenes with halogens
Eg bromine
. Halogen molecules act as electrophiles
. Bromine is likely to have an instantaneous dipole so Br f+ and Br f- due to VDW
. So the f+ end of one dipole is attracted to the electron rich double bond in the alkene
. The electrons in the double bond are attracted to the Br f+, and they repel the electrons in the Br-Br bond
. So this strengthens the dipole of the bromine molecule
. Two of the electrons from the double bond form a bond with Br f+, and the other bromine atom becomes a Br- ion
. So this leaves a carbocation, where the carbon atom that hasn’t bonded to the bromine has a positive charge
. So the :Br- ion now forms a bond with that carbocation
What is the product of alkenes with halogens
Di-halogenoalkanes are formed
What two steps are involved in the electrophilic addition of halide and ethene
. Formation of carbocation by electrophilic addition
. Rapid reaction with a negative ion
How do you test for a double bond
. Add a few drops of bromine water to an alkene
. It will turn from orange to colourless
This is because electrophilic reaction will occur which has colourless products
What are the conditions for reaction of ethene react with concentrated sulphuric acid
. Room temperature, and concentrated H2SO4
How does the reaction between ethene and concentrated sulphuric acid occur
What is the electrophile in it
- The electrophile is the positively charged hydrogen atom in the sulphuric acid molecule
- As the H f+ on it will attract electrons from the double bond, breaking it
. The bond between H and O in sulphuric acid will break as both electrons will go to the O
. Then, the H will bind to one carbon, so the other carbon in it will be a carbocation
. The :O-SO3H will bind to the carbocation
. So the product will be
ethyl hydrogensulfate
What happens when water is added to ethyl hydrogensulfate
An alcohol is formed and sulphuric acid reforms
So ethanol and sulphuric acid are the products
As the hydrogen from water reacts with the O-SO3H to make sulphuric acid
And the OH binds in place of the
O-SO3H
What is the structure of sulphuric acid
Here
O
=
H - O - S - O - H
=
O
