chapter 13 Flashcards
1
Q
describe an alkene
A
- they are unsaturated and contain carbon-carbon multiple bonds
- they have a functional group (c=c)
2
Q
how do you name an alkene
A
- identify the longest continuous carbon chain
- add the suffix -ene
- add any side chains
- use numbers to indicate where the double bond is
3
Q
how do you name an alkene when they have multiple double bonds
A
- 2 double bonds = diene
- 3 double bonds = triene
- 4 double bonds = tetraene
4
Q
explain the nature of a c=c double bond
A
- 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
5
Q
what is the 3d shape around the bond of an alkene
A
- there are three regions of electron density around each carbon atom
they repel each other as much as possible - creates the trigonal planar shape
6
Q
what is stereoisomerism
A
- 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
7
Q
what is the cahn-ingold-prelog priority rules
A
- 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
8
Q
explain the reactivity of the alkenes
A
- 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
9
Q
what are addition reactions
A
- 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
10
Q
explain the hydrogenation of alkenes
A
- alkenes react with hydrogen in the presence of a nickel catalyst
- forms alkanes
- the hydrogen molecule adds across the double bond
11
Q
explain the halogenation of of alkenes
A
- alkenes react with the halogens, chlorine and bromine, at room temperature
- forms dihaloalkanes
12
Q
how do you test for the presence of a c=c double bond
A
- add bromine water dropwise
- the orange colour will disappear (the bromine is decolourised)
13
Q
explain the reaction of alkenes with hydrogen halides
A
- 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
14
Q
explain the hydration reactions with alkenes
A
- 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
15
Q
explain the mechanism for halogenation with hydrogen halides
A
- 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
16
Q
what is Markownikoff’s rule
A
- 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
17
Q
A
18
Q
what is an electrophile
A
- high positive charge
- attracts and accepts a pair of electrons
- contains a positive dipole
- electron pair acceptor
