Organic Chem: Alkanes Flashcards
(26 cards)
What are alkanes?
Straight or branched chain hydrocarbons with only single covalent bonds between the carbon atoms
CnH2n+2
What do successive compounds in the alkane series differ by? What type of series is this?
One carbon and two hydrogen atoms
Homologous series
What are the two main sources of alkanes?
Natural gas
Petroleum
How are alkanes released as natural gas? What type is the main constituent?
Anaerobic decay of plant and animal matter by microorganisms
Methane (85-90%)
How is petroleum formed and what is it composed of?
Formed form the decomposition of plants and animals over millions of years
Composition varies but contains a mixture of hydrocarbons with smaller amounts of sulphur and nitrogen containing organic compounds
How is petroleum separated?
Fractional distillation
How to name an alkane
- Select longest continuous chain as parent, all other alkyl chains as branches
- Number carbon atoms in parent chain starting with end closest to the first carbon with branch
- Name each branch alkyl group and designate position on parent chain e.g. 2-methyl is methyl group attached to carbon 2
- When several different alkyl groups attached list alphabetically
- When same alkyl group occurs more than once indicate with prefix like di, tri, etc
What is an isomer?
When two molecules have the same molecular formula but different 3D structure
What is a constitutional/ structural isomer?
When molecular formulae are the same but the arrangement of atoms differ
Compare branched alkanes to straight chain alkanes.
Branched chains have smaller surface area and don’t pack together as well
Branched chains have fewer molecular interactions so lower boiling and melting points compare to straight chains
What are some physical properties of straight chains alkanes?
The longer the chain, the more interactions (van der Waal forces) so more energy is required to disrupt
At room temp: small molecules= gases
Larger molecules= liquids or solids
What are cycloalkanes?
Hydrocarbons forming a ring structure
Not planar
What are the two configurations of cyclic compounds?
Chair: all 6 conformations are gauche (skew). Energetically more favourable
Boat: 4 gauche and 2 eclipsed conformations
What is cracking?
The breakdown of a large alkane into smaller alkanes and alkenes
Small alkanes and alkenes are more usefully than long ones
What is dehydrogenation?
Removal of H2 to form unsaturated hydrocarbons
What is halogenation?
Replacing a H with a halogen
Radical reaction
Alkyl halides are useful as intermediates for the manufacture of compounds
What is a primary carbon in an alkane?
A primary carbon is bonded to one other carbon
What is a secondary carbon in an alkane?
A secondary carbon has two carbons attached to it
What is a tertiary carbon in an alkane?
A tertiary carbon is bonded to three other carbons
How much more reactive are secondary carbons compared to primary carbons?
Secondary carbons are four times as reactive as primary carbons
How much more reactive are tertiary carbons compared to primary carbons?
Tertiary carbons are five times as reactive as primary carbons
How are radicals stabilised?
Hyperconjugation
As the radical forms, the carbon centre transforms from sp3 hybridised to sp2 hybridised
Delocalisation of the bonding electrons from carbon next door by overlap with partly filled p orbital
Net stabilising effect
How do primary carbons act in hyperconjugation?
One bonding pair stabilise lone electron
How do secondary carbons act in hyperconjugation?
Two bonding pairs stabilise lone electron
