alkenes- chapter 13

Question 1

how is a pi-bond formed

Answer 1

• by the sideways overlap of two p-orbitals, one from each carbon atom of the double bond
• each carbon atom contributes one electron to the electron paired in the pi-bond
• the pi-electron density is concentrated above and below the line joining the nuclei of the bonding atoms
• the pi-bond locks the two carbon atoms in position and prevents them from rotating around the double bond

Question 2

why is the shape around a double bond trigonal planar

Answer 2

• there are 3 regions of electron density around each of the carbon atoms
• the three regions repel eachother as far apart as possible, so the bond angle is 120
• all atoms are in the same plane

Question 3

what are stereoisomers

Answer 3

have the same structural formula but a different arrangement of atoms in the space

Question 4

why does stereoisomerism happen around double bonds

Answer 4

because rotation amount the double bond is restricted and the groups attached to each carbon atom therefore are fixed relative to each other

Question 5

what has to be there for E/Z isomerism to occour

Answer 5

a C-C double bond
different groups attached to each carbon atom of the double bond

Question 6

what is cis-trans isomerism

Answer 6

a form of E/Z isomerism where one of the attached groups on each carbon atom of the double bond must be the same

Question 7

what are the Chan-Ingold-Prelog rules

Answer 7

• 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 diagonally places across the double bond, the compound is the E isomer

Question 8

why are alkenes more reactive than alkanes

Answer 8

• presence of the pi-bond
• being on the outside of the double bond, the pi-electrons are more exposed than the electrons in the sigma bond
• a pi-bond readily breaks and alkenes undergo addition reactions relatively easily
• pi-bond is weaker than sigma bond as so is broken more easily

Question 9

what happens to the pi-bond in an addition reaction of an alkene

Answer 9

the addition of a small molecule across the double bond, causing the pi-bond to break and for new bonds to form

Question 10

what is hydrogenation

Answer 10

when hydrogen is added across a double bond

Question 11

what is halogenating of alkenes

Answer 11

rapid addition reaction with the halogens chlorine or bromine at room temperature

Question 12

what is the test for unsaturation

Answer 12

the reaction of alkenes with bromine can be used to identify if there is a C-C double bond present and the organic compound is unsaturated

when bromine water is added to an alkene, bromine adds across the double bond
the orange colour disappears, indication the presence of a C-C double bond
it is decolourised if there’s no C-C double bond

Question 13

what is the addition reactions of alkenes with hydrogen halides

Answer 13

react at room temperature together to form haloalkanes
if the alkene is a gas= reaction takes place when the two gases are mixed
if the alkene is liquid= the hydrogen halide is bubbled through it
alkenes also react with concentrated HCl or hydrobromic acid, which are solutions of the hydrogen halides in water

Question 14

what is the hydration of alkenes

Answer 14

alcohols are formed when alkenes react with steam, in the presence of a phosphoric acid catalyst
steam adds across the double bond
the addition reaction is used widely across industry to produce ethanol from ethene
2 possible products