Alkenes Flashcards
What are alkenes?
Alkenes are unsaturated hydrocarbons with at least one C=C double bond.
What are some differences between alkenes and alkanes?
1) Aliphatic alkenes with one double bond have the general formula CnH2n
2) Alkenes are more reactive than alkanes, and typically take part in addition reactions.
3) Alkenes can form E/Z isomers, including cis-trans isomers.
What is the functional group in alkenes and what is it responsible for?
The C=C double bond is the functional group in an alkene and is responsible for its reactions.
What are the two parts that form the C=C double bond?
The C=C double bond is made up from two parts: a sigma bond and a pi bond. Sigma bonds are also present in alkanes.
What is a pi bond?
The pi bond is the reactive part of a double bond formed above and below the plane of the bonded atoms by sideways overlap of p-orbitals.
What happens in the formation of the double bond?
A sigma bond is formed directly between two carbon atoms by the overlap of orbitals.
A pi bond is formed above and below the plan of the carbon atoms by sideways overlap of p-orbitals.
Which bond prevents rotation of the double bond?
The pi bond fixes the carbon atom into postion.
In alkenes what bond are used in each carbon involved in a double bond?
1) Uses three of its electrons in the formation of three sigma bonds.
2) Uses one of its electrons in the formation of a pi bond.
What is the shape of alkene molecules and why?
There are three regions of electron density surrounding each carbon atom in the double bond.
Pairs of electrons repel each other as far apart as possible to minimise repulsions. The electron pairs repeal each other to give a trigonal planar shape, with bond angles of around 120.
What are the properties of cyclic alkenes?
1)Closed carbon rings with one or more double bonds.
What do the bond enthalpies of a C-C bond and C=C double bond show?
A double bond is stronger than a single bond and a pi bond is weaker than a sigma bond.
What happens to the carbon bonds when the alkene reacts?
The pi bond breaks and the sigma bond remains intact.
Describe what happens in the addition reactions of alkenes?
1) A small molecule is added across the double bond causing causing the pi bond to be broken.
2) Two reactant molecules react together to form one product molecule.
3) An unsaturated alkene reacts and forms a saturated product.
Explain the conditions and what happens when a mixture of hydrogen gas reacts with an alkene to form an alkane.
A mixture of hydrogen gas and a gaseous alkene passed over a catalyst of nickel at a temperature of 150 C. 1) The hydrogen adds across the double bond and an alkane is formed. 2) The reaction is sometimes known as hydrogenation and is an example of reduction.
Explain the conditions and what happens when alkenes react with halogens.
Alkenes react rapidly with halogens such as chlorine, bromine and iodine at room temperature.
1) The halogen adds across the double bond to give a di-substituted halogenalkane.
2) The reaction is sometimes known as halogenation.
