Alkanes and alkenes

• alkanes are non-polar molecules as C-C and C-H are not strongly polarised
• Intermolecular forces for alkanes are Van Der Waals - this allows for the BP and MP and solubility in water to be relatively low
• For alkanes, geometry at C is tetrahedral, trigonal planar or linear depending on the hybridisation and normal valency is assumed unless stated otherwise
• Geometric isomerism is a form of diastereoisomerism caused by the restricted rotation about the C=C
• Alkenes are generally prepared by elimination from alkyl halides or alcohols
• The Wittig reaction is used to couple two fragments together
• Alkenes are usually made by dehydration of alcohol instead of elimination of alkyl halides as alkyl halides are not easy to make
• In electrophilic addition, both C change from sp2 to sp3 when reacting with the E-X reagent
• Following Markonvnikovs empirical rule, addition of H is to the less substituted C of C=C - rationalised by considering which of the two possible intermediate carbenium ions will more readily form in theee rate limiting step
• For mechanism of electrophilic bromination:

  1. first Br+ forms intermediate bromonium ion - this bridges the carbons
  2. second Br- yield product - this attack from the opposite side of the carbon bridges (anti-addition)
• Anti-mechanisms has stereochemical implications as only the trans-diastereoisomer is formed
• Regioselectivity is when the bond formation is preferred at a particular atom over others
• Borane will add across C=C, the initial product is usually not isolated (alkyl borane), H2O2 is then added to form an alcohol
• For hydroboration, H2B+-H- is not highly polarised so the reaction mechanism is concerted - two parts add at the same time in one step.
• For the reaction of oxymercuration, mercuric acetate will add across C=C, acetate is a poor nucleophile to water attacks. NaBH4 converts the Hg-C bond to H-C
• Alkenes add H2 with the use of a catalyst which hydrogenates (reduces) other p-bond containing functional groups
• Heat of hydrogenation allows for relative stabilities of alkenes to be determined
• Infrared spectroscopy shows C=C weakly as it is not strongly polarised, =C-H is weak also. Bending modes below 1000cm-1 can be used to assign alkenes substitution
• NMR spectroscopy shows =C-H protons at 4.5-6.5ppm as these protons are in the C=C anisotropic de-shielding zone. J coupling patterns to other H are characteristic of the substitution pattern
• Simple alkenes do not show characteristic MS patterns
• In UV-Vis, Imax and E values increase steadily with increasing conjugation for alkenes and alkanes