Alkenes

Question 1

What is alkenes

Answer 1

• unsaturated hydrocarbons which contain one or more carbon double bond
• each C atom is sp² hybridised
• general formula: C_nH_2n
• geometry about each of the double bonded C is trigonal planar

Question 2

Why do cis isomers have higher boiling points that trans-isomers

Answer 2

• cis isomer is a polar molecule,
• C-Cl bond is polar and the dipoles of the 2 C-Cl bonds in cis-isomer do not cancel each other, molecule has a net dipole moment
• trans isomer is non-polar molecule
• dipoles of 2 C-Cl bonds in the trans isomer cancel each other, molecule has no net dipole moment
• more energy is needed to overcome stronger pd-pd attractions between molecules of the polar cis isomer Thant to overcome weaker id-id attractions between molecules of non-polar trans isomer

Question 3

Preparations of alkenes

Answer 3

• Cracking of alkanes
• Elimination of water from alcohols
• Elimination of halide from halogenoalkanes

Question 4

Saytzeff’s Rule(major/minor product from elimination reaction)

Answer 4

• most substituted alkene will be the most stable products and is the major product

Question 5

Chemical properties of alkenes(C=C double bond)

Answer 5

• C=C bond in alkene are electron rich due to the prescence of pie electron cloud
• pie electron provide reactive site which attract electrophiles or electrophillic reagents
• undergoes electrophilic addition

Question 6

What is Electrophilic addition (electrophile)

Answer 6

• electron rich molecules donates a pair of electrons to electrophiles
• electrophiles is a cation or electrically neutral molecule which has an electron deficient atom

Question 7

Reactions of alkenes

Answer 7

• Electrophilic addition of halogen
• Electrophilic addition of hydrogen halides
• Electrophilic addition of water/steam(hydration)
• Oxidation by KMnO₄
• Reduction via catalytic hydrogenation

Question 8

Markinov’s rule

Answer 8

• major product is one in which H atoms add to carbon of double bond having the greater number of hydrogen atoms
• carbocation with more electron-donating alkyl groups bonded to the electron deficient carbon can disperse the positive charge to a greater extent and will have a greater stability