Alkanes Flashcards
What are alkanes?
- straight branched hydrocarbon chains
- single covalent bonds between carbon atoms
- homologous series: successive compounds in the alkane series differ by 1 carbon and 2 hydrogens (CnH2n + 2 (n= number of carbons))
Sources of alkanes
natural gas
- anaerobic decay plant/animal matter via microbes
Petroleum
- decomposition of plants/animals over millions of years
- mixture of hydrocarbons with smaller amounts sulphur and nitrogen containing organic compounds: separated by fractional distillation
What are the uses of alkanes
- gas
- petroleum
- fuel
Alkanes in biology
- floral scents
- animal pheromones
- plant waxy cuticle
Naming system of alkanes
Prefix (where located) - parent (how many carbons) - suffix (what family does it belong to)
- alkanes suffix = ‘ane’
- parent = longest continuous carbon chain
- name each branch of alkyl group and position on parent chain ( e.g. 2-methyl = methyl group on carbon 2)
- name alkyl groups alphabetically (if branches the same indicate with prefix e.g. di, tri)
Chemical formula of methyl
CH3
Chemical formula ethyl
CH2CH3
Chemical formula propyl
CH2CH2CH3
isomers
= 2 molecules with the same molecular formula but with different 3D structures
Constitutional (structural) isomers
= 2 molecules with the same molecular formula but different arrangements of atoms
Branched alkanes properties
- smaller surface area = dont pack as well together
- fewer intermolecular interactions so lower boiling/melting points than straight chain
Straight chain alkane properties
At room temperature:
- small molecules = gasses
- larger molecules = liquids/solids
The longer the chain the more interactions (van der waals) = takes more energy to break it apart: higher melting/boiling point than branched chains
Conformation
The orientation of atoms in a molecule
What are the 2 readily available conformations of alkanes (when 2+ bond)
Eclipsed (the hydrogens almost align)
Staggered ( the hydrogens dont align and fill alternate spaces)
The parent name for alkanes (up to 6 carbons)
1 = meth 2 = eth 3 = prop 4 = but 5 = pent 6 = hex
Give the prefixes of alkanes (first 4)
Methyl
Ethyl
Propyl
Butyl
What prefix is used for hydrocarbon ring molecules
Cyclo
Stereoisomerism
When 2+ compounds have the same structural formula but differ in arrangement of the bonds in space (how rotations are formed around single bonds)
What are the 3 different types of stereoisomerism
- conformational
- geometric
- optical
When chlorine’s are added to an alkane what kind of conformations can be formed?
- fully eclipsed
- gauche (skewed)
- eclipsed
- anti (staggered)
Lowest energy when chlorine’s facing completely away from eachother
Cycloalkanes
- hydrocarbons form ring structure
- formula: CnH2n with sp3 hybridised carbons
- restricted rotation of c-c bonds = cyclic structures
2 configurations - chair (all 6 conformations gauche = more energetically favourable)
-boat ( 4 gauche and 2 eclipsed conformations)
Cis isomers
In cycloalkanes when 2+ substituents are pointing the same direction
Trans isomers
In cycloalkanes when 2+ substituents are pointing opposite directions
What are the 3 main reactions of alkanes
- combustion
- cracking
- dehydrogenation
- halogenation
Combustion
Alkanes react with oxygen to create carbon dioxide and water
- releases heat and have large negative enthalpies of combustion
Cracking
- large alkanes break into small alkanes and alkanes
- more useful when small
Dehydrogenation
- takes hydrogen molecule away fom alkane and to make an alkene
Halogenation
- replacing a hydrogen in an alkane with a halogen (happens in bright light) and forms a halogenoalkane = radical reaction
Starts chain reaction (3 stages: initiation, propagation, termination)
Explain the initiation stage of halogenation
- break the Cl-Cl bond = 2 Chlorine atoms
- is a homolytic break = one electron goes to each
- creates 2 free radicals ( very reactive)
Explain the propagation stage of halogenation
- Chlorine free radical takes hydrogen from methan to form hydrogen chloride
- creates methyl free radical
- methyl free radical reacts with a chloride = another free radical Coloring and molecule of chloromethane
This step can repeat over and over again
Explain the termination stage of halogenation
Free radicals are removed by either:
- 2 chlorine free radicals reacting to create chlorine
- 2 methyl free radicals reacting tom create ethane
- 1 chlorine free radical and methyl free radical react together to give chloromethane
What is a primary carbon
A carbon bonded to 1 other carbon
What is a secondary carbon
A carbon bonded to 2 other carbons
What is a tertiary carbon
A carbon bonded to 3 other carbons
What is the relative reactivity of primary : secondary : tertiary and why?
1 : 4 : 5
Caused by hyper conjugation
- forms a radical which is stabilised by hyperconjugation
- carbon centre changes from sp3 to sp2
- delocalisation of bonding electrons from carbon next door overlap and partially fill p orbital = net stabilising effect
The more binding pairs (prim, sec, tert) the H can contribute their electronegativity and make more stable
