12. Alkanes Flashcards
Why do longer chains have higher boiling points?
larger molecules = more electrons = more contact points = more LDFs = more energy to overcome them all = higher boiling point
What is a sigma bond?
the overlap of 2 orbitals, one from each bonding atom, directly in the plane of the 2 atoms, to form a covalent bond
it is positioned on a line directly between bonding atoms
Why do branched isomers have lower boiling points?
fewer surface points of contact = fewer LDFs = less energy needed to overcome
also: branches get in the way and stop molecules getting as close together
Why are alkanes used as fuels?
readily available, easy to transport, burn without releasing toxic products (in a plentiful supply of oxygen)
Why is incomplete combustion dodgy?
carbon monoxide = v toxic, binds to haemoglobin
carbon = soot, asthma, respiratory problems
Do alkanes require UV light to react?
Yep
What happens in initiation?
covalent bond in halogen is broken by UV light, each halogen atom takes one electron forming 2 v reactive free radicals
What happens in propagation?
chain reaction:
CH4 + Br2
1. Br radical reacts with C-H bond, forms methyl radical and HBr
2. methyl radical reacts w another Br2 molecule, forming organic product CH3Br and a new Br radical
cycle continues
terminated when 2 radicals collide
What happens in termination?
2 radicals collide, forming a molecule with all electrons paired.
both radicals are removed from the reaction mixture, so reaction stops
What are the limitations of radical substitution in organic synthesis?
further substitution (can continue until all Hs substituted for something else) substitution at dif. positions on carbon chain, exacerbated by further substitution
Why is the halogen bond and not the C-H bond broken when UV light is used?
bonding pair of electrons is further from the nuclei as halogen atoms are bigger, so less energy is needed to break the bond
How can you control further substitution?
use excess hydrocarbon to get more monosubstituted product
use excess halogen to get more polysubstituted product
Why is having a mixture of products bad?
requires fractional distillation to separate, which is costly due to energy consumption
Why is it difficult to produce a single organic product in free radical substitution?
mixed products due to:
multiple substitution of H
several isomers of longer chain haloalkanes produced
different products formed in termination step
Why don’t alkanes react with nucleophiles and electrophiles?
alkanes are non-polar and nucleophiles/electrophiles are only attracted to polar substances
also C-H bonds are strong
