Alkanes and alkenes

Question 1

What is an alkane?

Answer 1

A homologous series of hydrocarbons with single bonds.

Question 2

What is the shape of an alkane?

Answer 2

Tetrahedral, 109.5’

Question 3

How is a sigma bond formed?

Answer 3

Overlap of 2 orbitals, 1 from each bonding atom. Positioned on line directly between bonding atoms.

Question 4

How is crude oil separated?

Answer 4

By fractional distillation. Boiling point increases and chain length increases. London forces acting between molecules in close surface contact. The more contact points, stronger the London forces - boiling point increases.

Question 5

How is the boiling point of isomers affected?

Answer 5

Different boiling point but same molecular mass as branching increases, less points of contact, so weaker London forces.

Question 6

How do alkanes combust?

Answer 6

Complete - lots of oxygen present
Incomplete - not enough oxygen. Produces CO or C. CO is toxic as binds to Hb instead of O2 - oxygen deprivation.

Question 7

Are alkanes reactive?

Answer 7

Not very, as stable molecules due to only having single bonds.

Question 8

How do alkanes react with halogens?

Answer 8

With UV light for energy. Radical substitution reactions.

Question 9

What are the steps for radical substitution?

Answer 9

1. Initiation - produce free radicals
2. Propagation - free radicals used up/created in chains reaction
3. Termination - free radicals used up

Question 10

What is an alkene?

Answer 10

Homologous series of hydrocarbons with 1 or more carbon double bond

Question 11

How is the double bond formed?

Answer 11

Sigma bonds use up 3 electrons. 1 electron in the p orbital is left - a pi bond formed from the sideways overlap of 2 p orbitals. Density concentrated above and below the nuclei.

Question 12

What are the properties of a pi bond?

Answer 12

Fixes the 2 carbons in place and stops rotation. Much weaker as have electron density spread out so low bond enthalpy.

Question 13

What is the bond angle around the double bond?

Answer 13

120’ because 3 areas of electron density around each carbon.

Question 14

What is stereoisomerism?

Answer 14

The same structural formula with a different arrangement of atoms in space.
Forms because the double bonds cant rotate.

Question 15

What is E/Z isomerism?

Answer 15

Different groups attached to C=C.
E - same groups diagonally across c=c
Z - same groups across c=c.

Question 16

What reactions do alkenes undergo?

Answer 16

Electrophilic addition reactions

Question 17

How are alkenes hydrogenated?

Answer 17

H2, nickel catalyst, 150’C to form an alkane

Question 18

How are alkenes halogenated?

Answer 18

Rapid reaction at room temperature

Question 19

How are alkenes reacted with hydrogen halides?

Answer 19

Gaseous hydrogen halides at room temperature to form haloalkanes.

Question 20

How are alkenes hydrated?

Answer 20

Form alcohols when reacted with steam, phosphoric acid catalyst at 300’C and 150 atm.

Question 21

What is an electrophile?

Answer 21

An electron pair acceptor

Question 22

What is the test for alkenes?

Answer 22

Decolorisation of bromine water as it adds across the double bond.

Question 23

What is Markowninikoff’s rule?

Answer 23

When a hydrogen halide reacts with unsymmetrical alkene, the hydrogen attaches to the carbon atom with the most hydrogen atoms and the fewest carbon atoms.

Question 24

What 2 products can form from a reaction with an alkene?

Answer 24

The major or minor product. Major is more likely to be formed as the carbocation is more stable.

Question 25

What makes a carbocation more stable?

Answer 25

Electron donating ability of groups - push electrons to the positive charge of the carbocation. The more alkyl groups attached, mote charge is spread out to increase the stability.

Question 26

How are polymers formed?

Answer 26

Alkenes can undergo addition polymerisation to form polymers.
eg. poly(ethene), poly(propene).
poly(tetrafluoroethene) aka teflon

Question 27

What are environmental issues with polymers?

Answer 27

They are cheap and convenient, non reactive and non biodegradable. Recycling means they are sorted by type.
- PVC - hazardous as Cl2 is burnt –> HCl. Solvent to dissolve
- Fuel - high stored energy value so make electricity when burnt
- feedstock recycling - reclaim monomers for raw materials.

Question 28

What are bioplastics?

Answer 28

They are made of plant starch/cellulose. Conserve natural oil use as are broken down by microorganisms. Compostable polymers degrade.

Question 29

What are photodegradable oil based polymers?

Answer 29

They can be broken down by absorbing sunlight.