Organic chemistry- alkanes Flashcards
describe the bonding in alkanes
hydrogen and carbon atoms are covalently bonded. Each carbon atom has 4 sigma bonds.
how is a sigma bond formed?
a sigma bond is a type of covalent bond. It is caused by the overlapping of two orbitals
describe the shape of alkanes
four electron pairs in a sigma bond causes repulsion resulting in a tetrahedral arrangement where the bond angles are 109.5 degrees.
how is crude oil separated into different fractions?
fractional distillation in a distillation tower. This is possible as the different alkanes have different boiling points.
why does the length of the chain effect the boiling point?
the longer the chain, the higher the boiling point as the surface area is greater, more London forces will occur.
what is the effect of branching on alkanes?
the greater the branching, the fewer the surface CONTACT so less London forces will occur
why don’t alkanes react with most common reagents?
- sigma bonds are strong
- C-C bonds are non-polar
- the electronegativity of carbon and hydrogen are so similar that the C-H bond can be considered non-polar
why are alkanes commonly used as fuel?
- combustion of alkanes give out heat
- easy to transport
- no toxic products
complete combustion of alkanes forms?
carbon dioxide and water
incomplete combustion of alkanes forms?
carbon monoxide and water. Further limited oxygen forms C and water.
what is needed when alkanes react with halogens?
UV radiation. It provides energy for homolytic fission.
interactions of alkanes and halogens are examples of substitution reactions and radical subsitution. What are the three stages for this mechanism?
initiation, propagation and termination
what occurs during the initiation stage in free radical substitution?
the covalent bond of the halogen molecule is broken by homolytic fission, two highly reactive halogen radicals are formed.
what is a radical?
a highly reactive species with an unpaired electron.
what occurs during the propagation stage?
a chain reaction occurs through two main propagation steps. The two steps reoccur in a cycle.
what occurs during the first propagation step? example: bromination of methane
the bromine radical will react with the C-H bond in the methane (CH4) to produce a methyl radical (CH3) and a molecule of hydrogen bromide (HBr).
CH4 + Br. -> .CH3 + HBr
what occurs during the second propagation step?
the methyl radical will react with another bromine molecule, forming the organic product bromomethane, CH3Br, with a new bromine radical.
.CH3+Br2 -> CH3Br + Br.
what occurs in the termination step?
two radicals collide forming a molecule with all electrons paired.
what are the limitations of free radical substitution?
- further substitution occurs, creating a variety of products
- substitution can occur at different positions in the carbon chain
when are sigma and pi bonds formed?
sigma bonds are present in the first bond. Pi bonds are present in any second or third bond.
how is a pi bond formed?
by the unhybridized p-orbitals that exist above and below the sigma bond.
what two stages are needed to change alkanes into alkenes?
fractional distillation and cracking
What is the general formula for cycloalkanes?
CnH2n
as carbo atoms in cycloalkanes increase, why does the boiling point increase?
more surface points of contact for London forces to occur which requires more energy to overcome.
