ALKENES

Question 1

What are alkenes

Answer 1

unsaturated hydrocarbons

at least one double bond

Question 2

Why are alkenes more reactive than alkanes

Answer 2

they have double bonds which have a high electron density so can undergo addition reactions

Question 3

What are sigma bonds

Answer 3

the overlap of 2 S orbitals, they align horizontally to form a single covalent bond

there is a strong electrostatic attraction between the nuclei and the shared pair of electrons

Question 4

What is a pi bond

Answer 4

the parallel overlap of 2 p orbitals

when the p orbitals merge they form 2 oblong shapes, one on the top and one on the bottom

Question 5

Why are pi bonds weaker than sigma bonds

Answer 5

the electron density is spread out above and below the nuclei, electrostatic attraction is lower as in sigma bonds the electron density is directly between the nuclei

Question 6

What does a double bond consist of

Answer 6

a pi bond and a sigma bond

Question 7

Stereoisomers definition

Answer 7

stereoisomers have the same structural formula but a different arrangement of atoms in space

Question 8

When do you get E/Z isomerism

Answer 8

when you have two different atoms or groups of atoms on the same carbon in an alkene

Question 9

Rules of naming E and Z isomers

Answer 9

Label carbons with the double 1 and 2

Find the atomic number of the element DIRECTLY BONDED TO THE C=C, the highest atomic number is given priority (if the element is the same, look further down the chain)

Use the highest priority group ON EACH CARBON to figure out if its E/Z

Question 10

What is an electrophile and why do they attack alkenes

Answer 10

An electron pair acceptor

they are deficient in electrons so are attracted to the double bond

Question 11

Electrophile examples

Answer 11

Positive charge ions - NO2+, H+

Polar molecules - HBr, H2SO4

Question 12

How to convert ethene to ethane

Answer 12

Add hydrogen gas at 150 degrees with a nickel catalyst

Question 13

Test for alkenes

Answer 13

decolouration of bromine water

from brown-orange to colourless

Question 14

How does bromine react with alkenes

Answer 14

As Br-Br approaches the alkene it becomes polarised as electrons repelled away from the electron dense pi bond

an electron pair in the double bond is attracted to the positive dipole and forms a bond breaking the Br-Br bond

this forms a carbocation intermediate, the remaining bromine atom will have a lone pair attached which will form a bond with the positive carbon

Question 15

How can alcohols be produced from alkenes

Answer 15

Steam and an acid catalyst

Question 16

How to make ethanol from ethene

Answer 16

reacting with steam and phosphoric acid catalyst, 300 degrees and 60 atm of pressure

any unreacted alkene is recycled through the process

Question 17

How are halogenoalkanes formed from alkenes

Answer 17

Electrophilic addition of hydrogen halides, follows the same mechanism as halogens as Hydrogen halides are permanently polarised

Question 18

How can unsymmetrical alkenes produce multiple types of product with hydrogen halides and which type is formed more often

Answer 18

as the hydrogen can form a bond with either carbon on the double bond, leaving either a primary or secondary carbocation intermediate

the secondary carbocation intermediate will form more often as will be more stable, the halogen will then bond with this leaving the final product

Question 19

What is markownikoffs rule

Answer 19

the major product when adding a hydrogen halide to an unsymmetrical alkene is where the hydrogen adds to the carbon with the most number of hydrogens attached already

Question 20

What are the two types of polymer

Answer 20

natural - proteins and natural rubber

synthetic - poly(ethene) poly(propene)

Question 21

Properties of poly(alkenes)

Answer 21

saturated molecules

non polar

therefore unreactive

don’t degrade well in landfill