alkenes

Question 1

explain the shape and bond angle around each carbon atom in a C=C double bond

Answer 1

Shape: trigonal planar
Bond angle: 120°
This is because there are three regions of electron density around each carbon atom which repel each other as far as possible

Question 2

What is an alkene?

Answer 2

An unsaturated hydrocarbon containing a C=C bond, which contains pi bonds and sigma bonds. The pi bond causes a lack of rotation around the double bond.

Question 3

What is a pi bond?

Answer 3

Sideways overlap of p orbitals

Question 4

What is a sigma bond?

Answer 4

Direct overlap of p orbitals

Question 5

What is a stereoisomer?

Answer 5

Compounds with the same structural formula but different arrangement in space

Question 6

Explain how a pi bond is formed

Answer 6

-for each C atom of the double bond, 3 electrons are involved in sigma bonds, and 1 is not. this electron is in a p orbital.
-the two p orbitals (one from each carbon) overlap and form a pi bonds, with electron density concentrated above and below the sigma bonds joining the two nuclei of the bonding atoms (above and below the line)

Question 7

what is E/Z isomerism?

Answer 7

An example of stereoisomerism, in terms of restricted rotation about a double bond

Question 8

What are the conditions for E/Z isomerism?

Answer 8

• A c=c double bond
  -Different groups attached to each carbon atom of the double bond

Question 9

How do we name E/Z isomers?

Answer 9

• If the groups of higher priority are on the same side of the double bond, the compound is the z isomer
• If the groups of higher priority are on diagonally placed across the double bond, the compound is the E isomer

Question 10

How do we assign priority using the Cahn-Ingold-Prelog nomenclature?

Answer 10

The higher the atomic number, the higher the priority (if the atoms are the same then you need to find the first point of difference)

Question 11

What is cis-trans isomerism?

Answer 11

A special case of e/z isomerism, where the conditions are the same except in cis-trans isomerism one of the attached groups on each carbon atom of the double bond must be hydrogen

Question 12

How do we name cis-trans isomers?

Answer 12

Cis= hydrogen atoms on the same side of the molecule (Z)
trans= hydrogen atoms diagonally opposite each other (E)

Question 13

What is an electrophile?

Answer 13

An election pair acceptor

Question 14

What is the word equation for hydrogenation of alkenes? Include any conditions and reagents

Answer 14

Alkene + H2 ———> alkane
reagents/conditions: Nickel catalyst, 150°

Question 15

What is the word equation for hydration of alkenes? Include reagents/conditions

Answer 15

Alkene + H2O(g) ———> alcohol
H3PO4 catalyst, H2O as steam

Question 16

What is the word equation for halogenation of alkenes using X2? include reagents/conditions

Answer 16

Alkene + X2 (eg Cl2) ———> dihaloalkane (e.g. 1,2-dichloropropane)

Question 17

What is the word equation for halogenation of alkenes using HX? include reagents/conditions

Answer 17

Alkene + HX (eg HCl) ———> haloalkane

Question 18

Name an example of an electrophilic addition reaction

Answer 18

but-2-ene and hydrogen bromide -> 2-bromobutane

Question 19

Describe the mechanism for electrophilic addition using but-2-ene and HBr as an example

Answer 19

1. Bromine more electronegative than hydrogen so HBr is polar
2. the electron pair in the pi bond is attracted to the partially positive H atom, casing the double bond to break
3. a bond forms between the H atom and a C that was in the double bond
4. H—Br bond breaks by heterolytic fission, with both electrons going to the Br
5. A bromide (Br-) ion and a carbocation are formed (C+)
6. Br- reacts with carbocation to form the addition product

Question 20

describe the stability of primary, secondary and tertiary carbocations

Answer 20

carbocation stability 1°< 2°< 3° (due to electron-donating ability of alkyl groups, more alkyl groups means more charge is spread out, so more stable)

Question 21

How are carbocations classified? (1°,2°,3°)

Answer 21

Dependent on the number of alkyl groups attached to the carbocation (primary = one alkyl group etc)

Question 22

what is markownikoff’s rule?

Answer 22

when a hydrogen halide is added to an unsymmetrical alkene, the major product is formed via the most stable carbocation

Question 23

Describe the method of addition polymerisation

Answer 23

X. Y. X. Y
. / |. |
n C=C ———————>. -[C-C]- n
/. . |. |
W. Z. W. Z
using a high temperature and pressure

Question 24

name some examples for methods of disposing waste polymers

Answer 24

recycling, pvc recycling, use as fuel, feedstock recycling

Question 25

name some examples for disposing of bio- and photodegradable polymers

Answer 25

breakdown by microorganisms into H2O, CO2 and biological compounds (biodegradable)
breakdown by light (photodegradable)