Core organic (Module 4) - Alkanes & alkenes
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How is a sigma bond formed in an alkane?
1 p orbital in each carbon atom overlaps to form the bond
As carbon chain length increases, state and explain the trend in boiling points of alkanes.
• Boiling point increases. • As chain length increases, the molecule has more electrons. This then causes the strength of London Forces to increase as more induced dipoles are formed. The stronger London Forces require more energy to break, increasing the boiling point.
As branching increases, state and explain the trend in boiling points of alkanes.
• Boiling point decreases. • As branching increases, the molecule becomes more spherical. This leads to lower surface contact between alkane molecules, weaker London Forces so those forces require less energy to break.
Explain why alkanes are unreactive molecules.
The C-C and C-H bonds have a high bond enthalpy and the sigma bond has very low polarity.
What is the definition of a fuel?
A substance that releases energy when burnt
What are the products formed when an alkane is completley combusted?
Carbon dioxide and water
What are the products formed from an incomplete combustion of an alkane?
Carbon, carbon monoxide and water
What is the reaction type and reagents required for adding a halogen to an alkane?
• Free radical substitution • Halogen (X₂) • UV light
What is meant by homolytic bond fission?
Where a covelant bond is broken and 1electron is donated to each atom
What is meant by a free radical?
A reactive species that possesses an unpaired electron
What is the method to write a free radical substitution mechinism?
• Initiation. Form 2 halogen radicals from the halogen by reacting it with UV light. • Propagation step 1. React the alkane with 1 halogen radical. This forms a hydrogen halide (HCL, HBr, HI etc) and a radicalised alkane. 1 hydrogen is removed from the alkane. • Propagation step 2. React the radicalised alkane with the halogen molecule (Cl₂, Br₂, I₂ etc). This forms the desired haloalkane and another halogen radical. • Termination. React radicals in propogation together so they can no longer form.
What is the general formula of an alkene?
CnH₂n
What bonds are made in a C=C bond?
1 sigma and 1 pi bond
Why are alkenes more reactive than alkanes?
• The C=C double bond is more electron dense than a C-C single bond. • Electrophiles, therefore, are more likley to attack that bond and react with the alkene.
Compare the relative strength between a sigma and pi bond in an alkene.
The pi bond is weaker
How does the pi and sigma bond form in an alkene?
• Pi bond. P orbitals overlap on each carbon atom, forming a bond above and below the plane of the molecule. • Sigma bond. S orbitals overlap.
What is the definition of a stereoisomer?
Same structural and molecular formula but a different arrangment in space
What 2 conditions are needed to form a stereoisomer?
1) Restricted roation around the C=C double bond 2) There are 2 different groups attached to the double bond
How are E and Z stereoisomers identified?
• Split the molecule in two, horizonally, at the double bond. If both groups are on the same side, it is a Z isomer. • If they are on different sides, it is an E isomer.
Compare and explain the difference in the boiling points of stereoisomer alkenes.
• Z isomers will have a higher boiling point than E isomers. This is because the molecule is polar. The electronegitive groups being on the same side cause the molecule to become slightly negative. This then forms both permanent dipole interractions and London Forces. • E isomers have a lower boiling point because as the electronegitive groups are on opposite sides of the molecule. This causes the dipoles to cancel out, only leaving London Forces. This causes the boiling point to decrease as London Forces require less energy to overcome.
What is meant by an addition reaction?
A reaction where 2 molecules react together to produde 1
State the reagent(s), condition(s) and type of reaction for an alkene -> alkane synthesis.
• H₂ • Nickel catalyst • Addition or reduction
State the reagent(s), type of reaction and product formed when an alkene reacts with a halogen.
• Elemental halogen. Cl₂, Br₂, I₂ etc • Electrophillic addition • Di-haloalkane
State the trend in stability of carbocation intermediates during electrophillic addition reactions of alkenes.
Trend: Stability increases as more carbons are present. Tertiary carbocations are most stable. Primary > Secondary > Tertiary