Alkenes

Question 1

3 steps for naming alkenes:-

Answer 1

1) choose end closest to the double bond and start numbering the longest chain.
2) if double bond is between between 2 and 3 write the lowest number.
3) write this number before -ene.
E.g 4-ethyl non-2-ene

Question 2

What is a sigma bond ?(sometimes shown as squashed-6 = sigma)

Answer 2

P-orbitals overlapping end to end

Question 3

What is a Pi-bond?

Answer 3

P-orbitals overlap side-to-side (weak bond)

Question 4

What makes up alkene double bond?

Answer 4

Sigma bond bonds two carbons, carbons have P-orbital visible above and below.

Pi-bond is formed by orbital side to side overlap (looks like two banana bonds around the sigma, between the Cs)

Question 5

Rotation:-

Answer 5

Free rotation about single bonds.

Lack of rotation about double bonds as would require breaking pi-bond= not enough energy.

Question 6

What is stereoisomerism?

Answer 6

Compounds with the same structural formula but a different arrangement in space.

Question 7

Preparation of ethene via catalytic dehydration:-

Answer 7

Ethanol -> ethene + H2O

Question 8

Cis:-

Answer 8

2 identical groups on same side of C=C

Question 9

Trans:-

Answer 9

2 identical groups of opposite sides of C=C

Question 10

Cahn-Ingold-Prelog rules (E/Z)

Answer 10

Prioritise the 2 groups attached to each C of C=C based on atomic number.

Question 11

Z:-

Answer 11

Top priority (atomic number) groups on same side. “Togezzer”

Question 12

E:-

Answer 12

Top priority groups on opposite side = E.

Question 13

Elimination (dehydration) making an alkene:-

Answer 13

Alcohol —> alkene + water

—-> = w/ concentrated H2SO4

Question 14

Sentence for ethanol making conditions:-

Answer 14

Steaming (steam) drunk at a 60th (pressure) birthday party, 300 people (warm room, 300oC) there drinking vodka (ethanol, alcohol made like this) and coke (contains phosphoric acid/ H3PO4).

Question 15

Heterolytic fission:-

Answer 15

Bond breaking, both electrons in the covalent bond go to just one of the bonded atoms.

Question 16

What does a curly arrow show?

Answer 16

Movement of a pair of electrons

Question 17

What is an electrophile?

Answer 17

Electron pair acceptor

Question 18

Part 1 of electrophilic addition:-

Answer 18

Alkene with 4 electrons in its double bond is approached by a bromine (Br2) molecule and the bromine’s electrons are repelled to the far end of the bromine molecule.

Question 19

Part 2 of electrophilic addition:-

Answer 19

Delta positive end of bromine molecule becomes an electrophile and so accepts electron pair from pi-bond. New covalent bond therefore created and so covalent bond in the bromine molecule is broken by heterolytic fission.

Question 20

Part 3 of electrophilic addition:-

Answer 20

Negative bromide ion formed is attracted to the carbocation (cat ion) left after the pi-bond broke and a new dihalogen alkene is formed.

Question 21

Markownikoff’s rule:-

Answer 21

When an unsymemetrical alkene reacts with a molecule e.g. HCl (g) the H atom of HCl goes to the double bonded carbon with the most H atoms already.

THE C WITH THE MOST H IN THE ALKENE GETS THE H WHEN H-X IS ADDED.

Question 22

Showing polymers:-

Answer 22

"n" of a monomer, bracketed = a repeat unit, 
Must be two carbons in visible width:-
   H       H
    |        |
--C----C--
    |        |
 CH3  CH3 

In brackets n
= poly(but-2-ene)

Question 23

How do you name polymers?

Answer 23

Monomer name in brackets, prefixed by poly e.g. poly(ethene)

Question 24

Alkene functionsl group:-

Answer 24

\ /
C===C
/ \