chapter 12 alkanes Flashcards
what are alkanes?
main component of crude oil and they are among the most stable organic compounds allowing them to remain on the earth for millions of years. they are saturated hydrocarbons containing only single covalent bonds
what is the general formula of alkanes?
CnH2n+2
what is a sigma bond?
a bond formed by the overlap of one orbital from each bonding atom, consisting of two electrons and with the electron density centred around a line directly between the nuclei of the two atoms
what is the bond shape/angle around carbon in an alkane?
109.5/tetrahedral
what is fractional distillation?
the separation of components in a liquid mixture by their different boiling points into fractions with different compositions.
what is the trend in boiling points of alkanes? why?
as chain length increases so does boiling point. as chain length increases there are more electrons meaning stronger LDF requiring more energy to overcome. they also have larger surface area so there are more contact points allowing more LDFs to arise.
what is the effect of branching on boiling points?
isomers have the same molecular mass but branched isomers have lower boiling points because there are fewer surface points of contact between molecules of the branched alkanes meaning less LDFs arise
why do alkanes tend to be unreactive?
•C-C and C-H sigma bonds are strong
•C-C bonds are non-polar
•the electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered to be non-polar
despite low reactivity what process are they good at carrying out?
combustion to produce CO2 and H2O in complete combustion
what are the products of incomplete combustion?
CO, H2O and sometimes C particulates(soot)
how do alkanes react with halogens and what do they form?
the reactants require high energy UV light (sunlight) for the reaction to take place and the products are a haloalkane and a hydrogen halide.
e.g. CH4 + Br2 = CH3Br + HBr
what type of reaction is halogen + alkane?
substitution because a H atom has been replaced by a halogen atom
what is an example of radical substitution?
bromination of methane
what are the 3 stages of the mechanism for halogenation of alkanes?
•initiation
•propagation
•termination
what happens at the initiation stage of bromination of methane?
•covalent bond in a Br2 molecule is broken by homolytic fission due to UV light
•this produces 2 highly reactive bromine radicals.
Br2 = 2Br•
what happens at the propagation stage in bromination of methane?
•in the first step a Br radical reacts with a C-H bond in methane forming a methyl radical and HBr
•in step 2 each methyl radical reacts with another Br2 molecule forming CH3Br and a new bromine radical.
1. CH4 + Br• = •CH3 + HBr
2. •CH3 + Br2 = CH3Br + Br•
Why is propagation a chain reaction?
the new Br radical formed in the second step reacts with a CH4 molecule as in the first step and these steps cycle until termination occurs.
what happens at the termination stage in bromination of methane?
•two radicals collide forming a molecule with all electrons paired
•this removes both radicals from the reaction stopping it from happening
1. Br• + Br• = Br2
2. •CH3 + •CH3 = C2H6
3. •CH3 + •Br = CH3Br
what are the limitations of radical substitution in organic synthesis?
•further substitution e.g. CH3Br becoming CH2Br2
•in bigger alkanes the substitution can happen at different points along the carbon chain.
