organic chemistry- alkenes and polymers

Question 1

what are the bond angles around alkenes and alkanes?

Answer 1

alkenes: 120
alkanes: 109.5

Question 2

unlike a sigma bond, why can a pi bond not rotate?

Answer 2

rotation would reduce the overlap of p-orbitals.

Question 3

why can stereoisomers be formed in only alkenes?

Answer 3

The structure of an alkene across the double bond is effectively locked.

Question 4

why are alkenes highly reactive molecules?

Answer 4

• double bond, bond enthalpy of the pi bond is less than the sigma bond as it is a sideways overlap of orbitals but sigma bonds directly overlap. Sideways overlapping makes the bond easier to break.
• higher electron density in the double bond

Question 5

geometrical isomers are?

Answer 5

cis and trans

Question 6

optical isomers are?

Answer 6

E and Z

Question 7

what are the two conditions needed for an alkene to have cis/trans isomers?

Answer 7

1) carbon atoms on double bond must be attached to two different groups
2) at least one of the groups should be the same on both carbon atoms of the double bond

Question 8

how is an isomer determined as cis or trans?

Answer 8

the molecules should be on the same side to be cis and the molecules should be on opposite sides to be trans.

Question 9

how is an isomer determined as E or Z?

Answer 9

the group that is the same side are Z and opposite is E.

Question 10

how do you use the E/Z system to name compounds with four different groups?

Answer 10

elements with the greatest atomic number are given the higher priority.

Question 11

What catalyst is needed in the hydrogenation of alkenes

Answer 11

H2 is added in an addition reaction with the presence of a nickel catalyst.

Question 12

how is unsaturation tested?

Answer 12

bromine water, positive result: colourless

Question 13

describe the addition reaction of alkenes with hydrogen halides?

Answer 13

Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes

Question 14

describe hydration reactions of alkenes?

Answer 14

alcohols are formed when alkenes are reacted with steam (H20) in the presence of a phosphoric acid catalyst

Question 15

what is an electrophile?

Answer 15

an electron pair acceptor.

Question 16

Describe how the electrophilic addition reaction between but-2-ene and hydrogen bromide takes place.

Answer 16

1) Bromine is more electronegative so H becomes a positive dipole and Br becomes a negative dipole.
2) The electron pair in the pi bond is attracted to the partially positive hydrogen, causing the bond to break.
3) A bond between the hydrogen and carbon forms
4) The H-Br bond breaks by heterolytic fission, electron pair stays with Br.
5) Br ion and carboncation is formed which reacts to form the final product.

Question 17

Bromine is a non-polar molecule, so how does it become polar?

Answer 17

Br approaches the alkene, interacting with the electrons in the pi bond causing an induced dipole.

Question 18

where does the carbocation form in an unsymmetrical alkene?

Answer 18

form on the carbon that is bonded to more carbons

Question 19

what conditions are needed for addition polymerisation to take place?

Answer 19

high pressure and temperature

Question 20

uses of poly(chloroethene) (PVC)?

Answer 20

flexible and rigid, used to make pipes, films and sheeting

Question 21

uses of poly(propene)?

Answer 21

toys, packing crates, ropes

Question 22

uses of poly(phenlyethene) or poly(styrene)?

Answer 22

packaging materials, plates. It has thermal properties

Question 23

uses of poly(tetrafluoroethene)?

Answer 23

used as coating for non-stick pans, shoes, clothing

Question 24

what is a carbocation intermediate?

Answer 24

organic molecule with a carbon atom bearing a positive charge

Question 25

why aren’t there equal amounts of the two isomers formed in electrophilic addition of an asymmetrical alkene?

Answer 25

more of the major isomer is made as the carbocation is more stable as it is bonded to more alkly groups therefore it exists for longer

Question 26

What is unsaturated?

Answer 26

Presence of a carbon double or triple bond

Question 27

what toxic gas can form from the combustion of polymers with chlorine?

Answer 27

HCl

Question 28

how does hydrogen carbonate remove hydrogen chloride toxic gas?

Answer 28

through neutralisation