Alkenes Flashcards
(38 cards)
What are alkenes?
Unsaturated hydrocarbons
Made of carbon and hydrogen only and have one or more C=C double bonds
Why do the double bonds of alkenes make them more reactive than alkanes?
Because of the high concentration of electrons (electron density) between the carbon atoms
When are alkenes produced in large quantities?
When crude oil is thermally cracked
What is the general formula of alkenes?
The homologous series of alkenes with one double bond has the general formula CnH2n
Why can’t double bonds rotate?
As well as normal C—C single bond, there is a p-orbital (which contains a single electron) on each carbon.
These two orbitals overlap to form an orbital with a cloud of electron density above and below the single bond.
This is called a pi-orbital and its presence means the bond cannot rotate.
What 3 types of isomers can alkenes form?
Chain
Position
Geometrical
What is positional isomerism (alkenes)?
Double bond is in different positions.
The longer the carbon chain, the more possibilities there will be and therefore the greater the number of isomers.
What is geometrical isomerism (alkenes)?
A form of stereoisomerism.
The two stereoisomers have the same structural formula but the bonds are arranged differently in space.
It occurs only around C=C double bonds.
Draw Z-but-2-ene
P217
Do the double bonds affect physical properties of alkenes?
No
What intermolecular forces are present between alkene molecules?
van der Waals
Why are the physical properties of alkenes very similar to alkanes?
Because the only intermolecular forces present between the molecules are van der Waals forces
Are alkenes soluble in water?
No
Why are alkenes more reactive than alkanes?
The double bond(s)
It forms an electron rich area in the molecule, which can be easily attacked by positively charged reagents (electrophiles)
Most of the reactions of alkenes are ___________ ___________.
Electrophilic additions
As alkenes are ___________ they can undergo _________ reactions
Unsaturated
Addition
Define electrophile
An electron pair acceptor
Why are alkenes not usually used as fuels (despite their combustion properties)?
Because their reactivity makes them very useful for other properties
Electrophilic reactions
P220-223
What is Markovnikov’s rule?
When hydrogen halides add on to alkenes, the hydrogen adds on to the carbon atom which already has the most hydrogens
What is the positive inductive effect?
Alkyl groups have a tendency to release electrons
What is the result of the positive inductive effect?
This electron-releasing effect tends to stabilise the positive charge of the intermediate carbocation. The more alkyl groups there are attached to the positively charged carbon, the more stable the carbocation is.
What type of carbocation tend to be more stable?
A positively charged carbon atom which has 3 alkyl groups (tertiary carbocation) is more stable than one with 2 alkyl groups (secondary carbocation), which is more stable than one with just 1 (primary carbocation).
What addition reaction is used to test for a C=C double bond?
Alkene + Br2
When a few drops of bromine solution (bromine water - reddish brown) are added to an alkene, the solution is decolourised because the products are colourless.
