alkenes

Question 1

what are the basic traits of alkenes?

Answer 1

1. at least 1 c=c bond, unsaturated
2. carbon atoms in c=c bond are sp2 hybridised(trigonal planar)
3. c=c bond in alkenes consist of 1 strong sigma bond and 1 weaker pi bond
4. CnH2n.

Question 2

what types of isomerism can alkenes exhibit?

Answer 2

1. constitutional isomerism — functional group isomerism, chain isomerism and positional isomerism.
2. cis-trans isomerism

Question 3

How do the melting and boiling point of alkenes compare to that of corresponding alkanes?

Answer 3

1. lower
2. slightly smaller electron cloud which is less polarisable
3. slightly weaker intermolecular instantaneous dipole-induced dipole interactions between molecules in simple molecular structure
4. less energy needed to overcome.

Question 4

how do melting and boiling points of straight-chain alkenes change with molecular mass?

Answer 4

increase

Question 5

what determines the physical state of alkenes at room temperature?

Answer 5

the no. of carbon atoms present.

Question 6

how do the melting and boiling points of branched alkenes compared to straight alkenes?

Answer 6

1. lower melting and boiling points = more volatile
2. smaller surface area of contact between branched isomers
3. weaker intermolecular id-id interactions
4. less energy required to overcome

Question 7

what is formed during combustion reaction?

Answer 7

carbon dioxide and water

Question 8

determine the solubility of alkenes in water and organic solvents.

Answer 8

1. insoluble in water
2. soluble in non-polar solvents

Question 9

why can alkenes undergo electrophilic addition?

Answer 9

1. reactive by virtue of electron-rich c=c bond.
2. c=c bond easily accessible to approaching reactants given that the pi electrons are located above and below the plane of the bond.
3. pi electrons serve as a source to attrack electrophiles or induce dipoles in approaching molecules to form electrophiles.
4. since alkene molecules are unsaturated due to presence of c=c bond, alkenes usually undergo electrophilic addition reactions.

Question 10

what are the reagents and conditions involved in the addition of halogens?

Answer 10

halogen in organic solvent(CCl4) at room temperature.

Question 11

what is observation when Br2 reacts with alkenes?

Answer 11

orange-red bromine decolourises

Question 12

what to take note for mechanism drawing?

Answer 12

1. must write electrophilic addition
2. must indicate relative rate(slow first step, fast second step)
3. curly arrows
4. equation
5. lone pairs and charge
6. dipoles

Question 13

describe electrophilic addition of halogens in words.

Answer 13

1. the pi bond in (alkene) approaches (halogen) and induces a dipole.
2. the (halogen) atom with partial positive charge, acts as an electrophile.
3. the pi electrons in the c=c bond act as the nucleophile and attack partial positive (halogen) to form an unstable carbocation intermediate and a (halogen) ion.
4. the (halogen) ion attacks the carbocation intermediate to form (final product).

Question 14

describe the reagents and conditions required for addition of aqueous halogens and the observations observed for aqueous bromine.

Answer 14

1. aqueous halogen at room temperature.
2. orange bromine water decolurises.

Question 15

describe the electrophilic addition of aqueous halogens in words.

Answer 15

1. the pi bond in (alkene) approaches (halogen) and induces a dipole.
2. the (halogen) atom with partial positive charge acts as an electrophile.
3. the pi electrons in the c=c bond act as the nucleophile and attack the partial positive (halogen) atom to form an unstable carbocation intermediate and a (halogen) ion.
4. the H2O(in excess) acts as a nucleophile, competes with (halogen) ion to attack the carbocation intermediate and forms (final product).
5. any other nucleophile, if present in the reaction mixture, can also attack the carbocation intermediate to form other addition products.

Question 17

why is a certain product formed at a higher rate/ higher proportion/ the major product?

Answer 17

1. carbocation leading to the formation of the major product is tertiary/secondary/primary, and thus has more alkyl groups bonded to the positively-charged carbon compared to the minor product.
2. As there are more electron-donating alkyl groups bonded to the positively charged carbon, its positive charge is dispersed to a larger extent, resulting in this carbbocation to be more stable.

Question 18

describe the adition of steam(hydration).

Answer 18

1. alkenes are hydrated to yield alcohols
2. Reagents and conditions: H2O(g), heat in the presence of concentrated H3PO4 catalyst at high temperature and pressure.
3. no observable channge
4. not a chemical test

Question 19

describe reduction (catalytic hydrogenation)

Answer 19

1. alkenes are hydrogenated to form alkanes
2. reagents and conditons: H2(g) and Ni catalyst
3. reduction reaction
4. no observable changes
5. not a chemical test

Question 20

describe mild oxidation.

Answer 20

1. alkenes are mildly oxidised to form diots — contain 2 OH groups.
2. Reagents and conditions: cold, alkaline KMnO4. Alkali used is KOH(aq) or NaOH(aq).
3. oxidation.
4. general eq: (alkene) + [O] + H2O = final product.
5. observations: purple KMnO4 decolourises, brown ppt of MnO2 formed.

Question 21

describe strong oxidation — oxidation cleavage.

Answer 21

1. reagents and conditions: acidified KMnO4, heat under reflux, H2SO4(aq) used.
2. Heat using water bath
3. oxidation reaction.
4. Purple KMnO4 decolourises, effervescence of CO2(g) to give a white ppt with limewater(only if CO2 produced) .
5. products formed: ketone, carboxylic acid, CO2 and H2O.
   - H2CO3 and Ethanedioic acid further oxidised to CO2 and H2O
6. Oxidation of alkenes cannot take place with potassium dichromate(VI).

Temperature is the main factor that determines whether mild/strong oxidation will take place, as both take place in acidic and alkaline medium.

Question 22

describe the dehydration of alcohols.

Answer 22

1. reagents and conditons: concentrated H3PO4 catalyst, heat
2. type of reaction: elimination
3. the major product formed is the one with more akyl groups, more stable as a result.