Chapter 14 - Alkenes

Question 1

What is the shape and bond angles in an alkene

Answer 1

Alkenes are planar molecules with bond angles of roughly 120

Question 2

What properties does the double bond in an alkene contain

Answer 2

It is a double covalent bond that is electron rich and cannot rotate.

Question 3

why can’t a double bond in an alkene rotate

Answer 3

There are p orbitals on both carbon atoms with overlap to form a pi orbital meaning that the double bond cannot rotate (restricted rotation)

Question 4

what two types of isomers are present in alkenes

Answer 4

position isomers
Geometrical isomers

Question 5

what are position isomers

Answer 5

when the double bond is in different positions in the carbon chain. e.g. but-1-ene and but-2-ene

Question 6

What are geometrical isomers

Answer 6

geometrical isomers are a form of stereoisomers with the same molecular formula but different structural formulas as the bonds are arranged differently.

Question 7

what are the rules for E-Z isomerism

Answer 7

If the two atoms with the greatest atomic number are on the same side of the double bond they are Z isomers. If they are on opposite sides they are E isomers.

Question 8

what are some physical properties of alkenes

Answer 8

The only intermolecular forces are Van der Walls. The double bond does not greatly affect the boiling point so the properties are similar to alkanes. Alkene are not soluble in water.

Question 9

Are alkenes more or less reactive than alkanes

Answer 9

Although the bond enthalpy is higher in alkenes they are more reactive due to electrophiles

Question 10

What are electrophiles with examples

Answer 10

electron pair acceptors

halogens
hydrogen halides
sulfuric acid
water (phosphoric acid catalyst)

Question 11

picture of bonding in an alkene

Answer 11

Question 12

example of position isomers in butene (picture)

Answer 12

Question 13

example of geometrical isomers in butene (picture)

Answer 13

Question 14

what are the steps in elecrophilic addition

Answer 14

1. The electrophile is attracted to the double bond
2. electrophiles are positively charged and accept a pair of electrons from the double bond. The electrophile may be a positively charged area.
3. a positive ion is formed (carbocation)
4. a negeatively charged ion forms a bond with the carbocation

Question 15

what is hetrolytic bond breaking

Answer 15

when a cocalent bond breaks both electrons go the same atom and none of them to the other atom

Question 16

outline the mechanism between ethen and hydrogen bromide (HBr)

Answer 16

Question 17

what is the positive induvtive effect in carbocations

Answer 17

When an atom or group contributes electrons to form a bond and gains a partial positive charge. This means that the more alkyl groups attached to a carbon the more neutral its partial charge meaning it is more stable which is why a tertiary carbocation is more stable than a primary carbocation.

Question 18

what the the trend in stability between primary and tertiary carbocaation

Answer 18

Question 19

outline the mechanism between ethene and bromine (Br2)

Answer 19

Question 20

what is the test for a double bond

Answer 20

add bromine waater - orange brown –> colourless

Question 21

outline the mechanism of the electrophilic addition between sulfuric acid aand ethene

Answer 21

Question 22

what is the reaction between ethene and water when water acts as an electrophile and what are the conditions for the reaction

Answer 22

phosphoric acid catalyst

Question 23

what is the equation for the polymerisation of ethene

Answer 23

Question 24

what are some examples of natural polymers

Answer 24

proteins, cellulose, DNA

Question 25

examples of polymers with their uses

Answer 25

Question 26

what are addition polymers

Answer 26

Addition polymers are formed by the addition of some simple monomer units repeatedly.

Question 27

what are plasticisers

Answer 27

plasticisers are additives that get between polymer chains allowing them to slide across eachother modifying their properties making them more felxible