Alkanes. Flashcards
What are alkanes?
CnH2n+2.
Saturated hydrocarbons.
Single C-C bonds and C-H bonds as sigma bonds.
Bonding in alkanes.
Only C and H atoms joined by single covalent bonds.
Each C atom joined to 4 other atoms by single covalent bonds-sigma bonds(overlap of 2 orbitals, one from each bonding atom).
Each overlapping orbital has one electron.
Sigma bond has 2 electrons shared between bonding atoms.
Each C atom has 4 sigma bonds(C-C or C-H).
Sigma bond on line directly between bonding atoms.
Explanation of the shape of alkanes.
Each C atom surrounded by 4 electron pairs in 4 sigma bonds.
Repulsion between electron pairs=tetrahedral shape around each C atom.
Bond angle=109.5 degrees.
Sigma bond acts as axis which atoms can rotate freely.
Non-rigid shapes.
Free rotation of sigma bond.
Explanation of varying boiling points of alkanes.
Boiling points increase as alkanes chain length increases.
Higher chain length=greater IMFs(london forces)= more energy needed to break apart=higher boiling point.
Effect of chain length on boiling point.
London forces act between molecules that are close in surface contact.
Higher chain length=larger molecule surface area=more surface contact possible between molecules=London forces between molecules greater=more energy needed to overcome forces.
Effect of branching on boiling point.
Branched isomers have a lower boiling point than straight-chained isomers.
Fewer surface points of contact in branched alkanes=fewer/weaker London forces.
Branches get in way and prevent branched molecules being as close as straight chained ones, decreasing IMFs even more(London forces).
Reasons for the low reactivity of alkanes.
C-C and C-H sigma bonds are very strong.
C-C bonds are non-polar.
Electronegativity of C and H is so similar that the C-H bond is considered non-polar.
High bond enthalpy of bonds present= not often enough energy to react.
Complete combustion of alkanes.
Burn completely to form CO2 and water.
In plentiful oxygen supply.
Used as fuels.
Readily available. easy to transport, don’t release toxic products.
Incomplete combustion of alkanes.
Limited O2, not enough for complete combustion.
H atoms are oxidised to water.
Combustion of carbon may be incomplete=toxic CO or soot.
Reactions of alkanes w/ halogens.
In presence of UV.
Provides initial energy needed for a reaction to take place.
Substitution reaction.
Mechanism for the bromination of alkanes.
Radical substitution.
Step1:initiation.
Covalent bond in Br broken by homolytic fission.
Each Br takes of electron from the pair=2 highly reactive Br radicals.
Full stop=radical.
Br-Br–>Br.+Br.
Step2:propagation.
2 steps=chain reaction.
1:CH4+Br.–>.CH3+HBr
2:.CH3+Br2–>CH3Br+Br.
Br radical reacts w/ C-H in methane=methyl radical(.CH3) and hydrogen bromide HBr.
Each methyl radical reacts w/ another Br molecule=bromomethane+new Br radical.
New Br radical reacts w/ CH4 molecule like prop step 1 and 2 steps continue to cycle through in chain reaction.
Continue until reactants used up/termination.
Step 3:termination.
2 radicals collide=molecule w/ all electrons paired.
Br.+.Br–>Br2 or
.CH3+.CH3–>C2H6 or
.CH3+.Br–>CH3Br.
When collide and react, both radicals removed from reaction mixture =stop reaction.
Limitations of radical substitution in organic synthesis.
Further substitution-
Another Br radical can collide w/ bromomethane molecule=subs another H molecule–>dibromomethane.
Can continue until all H have been subbed.
Substitution at different points in the carbon chain-
If carbon chain is longer, get a mix of monosubbed isomers by substitutions at diff pints of chain= different isomers formed.
