unsaturated compounds

Question 1

what are unsaturated compounds?

Answer 1

• contain double or triple bonds (alkenes and alkynes) -> electrons are more ‘exposed’ for reactivity (more reactive than alkanes)
• do not contain maximum number of hydrogens

Question 2

how to identify reduction/oxidation in unsaturated compounds

Answer 2

• more C-H bonds / less C=O-> reduction (oxidation of C decreases)
• less C-H bonds / more C=O-> oxidation (oxidation of C increases)

Question 3

what is an alkene?

Answer 3

C atoms in double bond are hybridised sp2 (trigonal planar arrangement

Question 4

describe the difference between E-Z (cis-trans) isomers

Answer 4

• caused by the rigid double bond which takes up the p orbitals to form pi bonds (refers to position of the substituent to double bond)
  
  • cis= same side
  • trans= opposite sides
• with asymmetric substitution a priority is assigned based on atomic number to each C atom
  
  • Z -> priority atoms are on the same side
  • E-> priority atoms are on opposite sides

Question 5

describe the reduction reactions of alkenes

Answer 5

addition of H (hydrogenation):

• proton (H+) is electrophile (attracted to e-) is attracted to pi orbital of free negative charge
  
  • pi orbital is further from nucleus and not shared as much as sigma bonds
  • one C is left with + charge and attract the hydride ion which acts as a nucleophile
• sequential addition of H+ or H- ions (proton + hydride ion)
• biological systems: H+ ions and hydride donating molecule (NADH) (lipid metabolism and biochemical pathways)
• lab system: H gas, catalyst (Pd/Pt), reactants (NaBH4)

Question 6

describe the electrophilic addition in alkenes

Answer 6

electrophiles use the electron rich orbital of the double bond to form bonds (HBr, H2O, etc…):

• 2 step mechanism with intermediate species
• acid catalyst added to increase mechanisms reactions (not consumed by the reaction, used and reproduced)
• asymmetric double bond: electrophile attacks the less substituted carbon (less C atoms bonded to it) to yield more stable cation
• stability: tertiary > secondary > primary

Question 7

describe polymerisation

Answer 7

creation of long C chains from alkenes through addition reactions of polymers

• high temperature and pressure
• catalyst
• radical reaction

Question 8

what are alkynes?

Answer 8

sp hybridised carbons (linear local arrangement)
- rotation is hindered in both bonds due to the p orbitals being occupied, forming the pi bonds around main orbital (sigma bonds)