Chapter 12 - Alkanes Flashcards
what is the general formula for alkanes?
CnH2n+2
what are covalent bonds in alkanes called?
sigma bonds
what is a sigma bond the result of?
the overlap of two orbitals, one from each bonding atoms
what shape is around each carbon atom in an alkane?
tetrahedral
what is the bond angles in alkanes?
109.5 degrees
why does boiling point increase with chain length?
the molecules have a larger surface area so more surface contact is possible between molecules resulting in greater London forces.
how do boiling points differ for branched isomers and straight chain isomers and why?
branched isomers have lower boilings points because there are fewer surface points of contact between molecules and branches prevent branched molecules from getting close together. these both reduce london forces.
why are alkanes unreactive?
C-C bonds and C-H sigma bonds are strong and C-C bonds are nonpolar
in the presence of what do alkanes react with halogens and why?
sunlight because the uv radiation provides the initial energy for the reaction to take place
what are the three stages of the mechanism for the bromination of methane?
initiation, propagation and termination
what happens during initiation?
the covalent bond in a bromine molecule is broken by homolytic fission, each bromine atom takes one electron from the pair forming two highly reactive bromine radicals
what happens during the first propagation step?
step 1 - the bromine radical reacts with a C-H bond in the methane forming a methyl radical and hydrogen bromide
what happens during the second propagation step?
step 2 - each methyl radical reacts with another bromine molecule forming bromomethane and a bromine radical
what happens during termination?
two radicals collide forming a molecule with all electrons paired
what does further substitution involve?
another bromine radical can collide with a bromomethane molecule substituting a further hydrogen hydrogen atom to form dibromoethane
