alkanes Flashcards
(27 cards)
source of alkane
crude oil
are alkanes un/saturated
saturated
both C-C and C-H bonds are made up of a ______ bond
sigma bond
describe the bonding in alkanes
alkanes are saturated hydrocarbons
both C-C and C-H bonds are made up of a sigma bond
the sigma bond is formed between two carbon atoms by the direct single overlap of orbitals directly between bonding atoms
this allows free rotation of the sigma bond
explain the shape and bond angle around each carbon atom in alkanes in terms of electron pair repulsion
tetrahedral shape with bond angle of 109.5
four bond pairs of electrons around C atom repl each other equally
carbon chain length
as the C chain length increases the boiling point increases
because there is more surface contact between molecules (and more electrons)
so there are more induced dipole-dipole interactions between the molecules
which need more energy to overcome
branching
a branched isomer has a lower boiling point than the unbranched isomer
in a more branched alkane, there is less surface contact between molecules leading to fewer induced dipole-dipole interactions between molecules
which need less energy to break the weaker induced dipole-dipole interactions between molecules
alkanes are relatively unreactive because…
the C-C and C-H sigma bonds are
non polar
strong
alkanes have high bond enthalpy which…
gives them a low reactivity
combustion
combustion of alkanes is an exothermic reaction
because heat energy is released to the surroundings
alkanes are useful as fuels
in a plentiful supply of oxygen alkanes burn completely to produce CO2 and H2O
in a limited supply of oxygen alkanes burn incompletely to produce CO and H2O
carbon monoxide
a toxic gas that prevents hemoglobin in red blood cells binding with oxygen which can be fatal
radical substitution facts
reagents - halogen (cl2 or br2 or i2) and excess alkane
conditions - uv radiation
radical substitution reaction
substitution type reaction because a H atom in the alkane molecule is replaced by a halogen atom
during the reaction the…
Cl-Cl (or Br-Br or I-I) and C-H bonds break by homolytic fission to form radicals so this reaction is known as radical substitution
define radical
a species with an unpaired electron
further substitution can occur when…
another H atom is replaced by another halogen atom
limitations of radical substitution
further substitution means that a mixture of haloalkanes (chloromethane, dichloromethane, trichloromethane and tetrachloromethane) is produced until all of the hydrogen atoms have been replaced
there may be reactions at different positions in a carbon chain creating structural isomers (1-bromopentane, 2-bromopentane, 3-bromopentane
define reaction mechanism
shows the sequence of steps which make up the overall reaction
order of steps in radical substitution mechanism
- initiation
- propagation
- termination
describe the steps of radical substitution mechanism for methane and chlorine
- the reaction initiates by breaking the Cl-Cl bond by homolytic fission to make two chlorine radicals
the uv radiation provides the energy needed to break the Cl-Cl bond - a very reactive chlorine radical attacks a methane molecule in a chain reaction
the chlorine radical is used up in the first propagation step but is regenerated in the second propagation step (acts like a catalyst). The chlorine radical is now able to attack a new methane molecule and the reaction continues - propagates - the reaction terminates when any two radicals combine to form a new molecule.
a chlorine radical reacts with a methane molecule, removing a hydrogen to create a methyl radical and hydrogen chloride
propagation (1st step)
two methyl radicals react to form ethane
termination
uv light is absorbed by a chlorine molecule providing energy to break the bond. One electron goes to each chlorine creating two free radicals. his is homolytic fission
initiation
two chlorine radicals react to form chlorine
termination
