chapter 13

Question 1

describe an alkene

Answer 1

• they are unsaturated and contain carbon-carbon multiple bonds
• they have a functional group (c=c)

Question 2

how do you name an alkene

Answer 2

• identify the longest continuous carbon chain
• add the suffix -ene
• add any side chains
• use numbers to indicate where the double bond is

Question 3

how do you name an alkene when they have multiple double bonds

Answer 3

• 2 double bonds = diene
• 3 double bonds = triene
• 4 double bonds = tetraene

Question 4

explain the nature of a c=c double bond

Answer 4

• sp2 hybridisation
• has one remaining electron that stays in the p orbital
• this creates 3 bonds
• the sideways overlap of p orbitals is called a pi bond - in one double bond, there is one sigma bond and one pi bond

Question 5

what is the 3d shape around the bond of an alkene

Answer 5

• there are three regions of electron density around each carbon atom
  they repel each other as much as possible
• creates the trigonal planar shape

Question 6

what is stereoisomerism

Answer 6

• they have the same structural formula but different arrangements of the atoms in the 3d shape
• e-z isomerism only occurs in compounds that have c=c bonds and has two different groups attached to each of the c=c double bonds
• optical isomerism

Question 7

what is the cahn-ingold-prelog priority rules

Answer 7

• the atoms attached to the carbon atoms in the c=c bond are given priority based on their atomic numbers
• if the groups of higher priority are on the same side of the double bond, it is a Z-isomer
• if the groups of higher priority are on different sides of the double bond, it is an E-isomer
• if the first two atoms attached to the double bond are the same, then the first point of difference needs to be found

Question 8

explain the reactivity of the alkenes

Answer 8

• much more reactive than the alkanes
• the pi bond electrons are more exposed than the sigma bond electrons as they are on the outside of the double bond
• the pi bond breaks easily and the alkenes can undergo addition reactions

Question 9

what are addition reactions

Answer 9

• two reactant molecules join together to form one product
• alkenes undergo them
• hydrogen in the presence of a nickel catalyst
• halogens
• hydrogen halides
• steam in the presence of an acid catalyst
• each reaction involves the addition of a small molecule across the double bond causing the pi bond to break and new bonds to form

Question 10

explain the hydrogenation of alkenes

Answer 10

• alkenes react with hydrogen in the presence of a nickel catalyst
• forms alkanes
• the hydrogen molecule adds across the double bond

Question 11

explain the halogenation of of alkenes

Answer 11

• alkenes react with the halogens, chlorine and bromine, at room temperature
• forms dihaloalkanes

Question 12

how do you test for the presence of a c=c double bond

Answer 12

• add bromine water dropwise
• the orange colour will disappear (the bromine is decolourised)

Question 13

explain the reaction of alkenes with hydrogen halides

Answer 13

• alkenes react with gaseous hydrogen halides at room temperature
• forms haloalkanes
• alkenes also react with concentrated hydrochloric or hydrobromic acid
• if the alkenes are unsymmetrical, two possible products are formed

Question 14

explain the hydration reactions with alkenes

Answer 14

• alkenes react with steam in the presence of of a phosphoric acid catalyst
• forms alcohols
• reaction is used widely in industry
• if the alkene is unsymmetrical, there are two possible products formed

Question 15

explain the mechanism for halogenation with hydrogen halides

Answer 15

• an electron pair in the pi bond is attracted to the delta positive charge on the hydrogen halide
• the hydrogen halide bond breaks through heterogeneous fission
• a halide ion and a carbocation intermediate
  form
• the halide ion will react with the carbocation and form the haloalkane

Question 16

what is Markownikoff’s rule

Answer 16

• the major product always forms via the most stable carbocation intermediate
• the stability increases as the as alkyl groups are added to the positively charged carbon atom

Question 18

what is an electrophile

Answer 18

• high positive charge
• attracts and accepts a pair of electrons
• contains a positive dipole
• electron pair acceptor