Alkenes 09D

Question 1

what shape is the ethene molecule?

Answer 1

trigonal planar with respect to each carbon atom

Question 2

what is the c=c double bond made up of?

Answer 2

1 sigma and 1 pi bond

Question 3

*in alkenes, what bonds do carbons form with each other?

Answer 3

sigma bonds by head on overlaps of sp2 orbitals. the unhybridised p orbitals form side on pi bonds with each other, perpendicular to the sigma head on bonds in carbon, forming a H shape. the electron cloud of pi bonds lie above and below the plane of atoms.

Question 4

*what are the requirements for cis-trans isomers to happen?

Answer 4

there has to be:

1. restricted rotation about the c=c double bond
2. 2 different groups has to be attached to each c=c double bond atom

Question 5

what’s the difference between cis and trans isomers?

Answer 5

cis: the same groups have to be on the same side of the c=c double bond
trans: the same group are on the opposite sides of the c=c double bond.

Question 6

restrictions of cis-trans isomers (ie what isomers cannot be cis-trans)

Answer 6

1. if one of the carbon atoms of the double bond are attached to both of the same groups (eg one carbon is attached to 2 methyl)
2. cycloalkanes with LESS THAN 7 CARBON ATOMS. cycloalkanes with less than 7 carbon atoms can only exist as cis isomers. However, cycloalkanes with more than 8 carbon atoms can have cis-trans isomers.

Question 7

physical properties of alkenes: boiling and melting points of alkenes

Answer 7

(same as alkanes) when the number of carbon atoms increase, the number of electrons and hence the size of the electron cloud increases. Therefore, there are stronger dispersion forces that require more energy to break apart.

Question 8

what is the difference between cis and trans isomers in bp and mp?

Answer 8

cis isomers has a higher boiling point.

cis isomers tend to be more polar, which means that pd-pd dipole interactions and dispersion forces both exist between molecules. meanwhile, trans isomers are non polar, meaning that they only have dispersion forces between the molecules, which require less energy to break.

trans isomers have a higher melting point.

the trans isomers pack better due to the MORE symmetrical shape of the trans isomer. The cis isomer has a U shape and packs less well. Hence, in trans isomers, dispersion forces work more effectively when they are more closely packed, and requires more energy to break apart.

Question 9

what are the ways to determine the melting or boiling point of cis and trans isomers?

Answer 9

1. consider their imf

2. consider the size of their electron cloud, respective to their proton number.

Question 10

why are alkENES more reactive than alkANES?

Answer 10

the c=c double bond is more rich in electrons.

Question 11

what reactions do alkenes undergo?

Answer 11

addition, reduction

Question 12

for ADDITION reactions with alkene and bromine, what are the requirements?

Answer 12

absence of UV light or heat, as substitution may occur with it

Question 13

what are the requirements for reduction reactions with alkenes? (addition of H2)

Answer 13

H2 gas, Ni catalyst, high pressure and room temperature.