Chemical Properties Of Alkenes (LEC)

Question 1

Alkenes are much more reactive than their corresponding alkanes. Why is this so?

Answer 1

Their greater reactivity is due to their Carbon=Carbon double bonds

Question 2

Alkenes burn in the air with a _____, forming CO2 and water as products of combustion

Answer 2

Luminous (yellow) flame

Question 3

In Combustion, what does the unsaturation of molecule result in alkenes (olefins) as compared to alkanes (palefins)?

Answer 3

• Despite an alkane having the same amount of C atoms as the alkene, the alkene will result in a greater percentage of Carbon
• The greater percentage of C in alkenes interacting with the flame produce more luminosity

Question 4

What result will Alkenes give in Baeyer’s test for unsaturation?

Answer 4

Alkenes will give a positive result

Question 5

What is the relationship of KMnO4 with Alkenes in Baeyer’s test?

Answer 5

• The KMnO4 will oxidize the alkene
• During oxidation, the permanganate ion (MnO4) is reduced and loses its purple color

Question 6

What will the permanganate ion (MnO4) be reduced to?

Answer 6

It will be reduced to the brown precipitate of MnO2.

Question 7

What else is formed during Baeyer’s test?

Answer 7

A diol, which is a chemical compound containing two hydroxyl groups (−OH groups)

Question 8

What are Addition Reactions?

Answer 8

• The most common way Alkenes react
• These reactions are the reverse of elimination reaction
• Double bond disappears

-C- = -C- + A-B -> A-C = C-B

Question 9

3 Types of Addition Reaction:

Answer 9

• Hydrogenation of Alkenes
• Halogenation of Alkenes
• Addition of Hydrogen Halides

Question 10

What is Hydrogenation of Alkenes?

Answer 10

• A reaction conducted by introducing an alkene to hydrogen gas (H2) with a metal catalyst

RCH=CH2 + H2 ->[metal catalyst] RCH2CH3

• The 2 H atoms replace the formed double bond, thus removing it

Question 11

Metal catalysts for Hydrogenation can be either:

Answer 11

• Platinum (Pt)
• Palladium (Pd)
• Nickel (Ni)

Question 12

What is Halogenation of Alkenes?

Answer 12

• The addition of Br2 or Cr2 to the alkene at room temperature
• Double bonds are replaced, and alkyl dihalides are formed

-C- = -C- + X2 -> CX-CX

Question 13

In halogenation, how is the reaction of Br with alkene qualitatively useful in ascertaining the presence of a double bond?

Answer 13

Proof of a double bond is visibly shown when the deep brown-red color of Br is discharged when Br is added to alkene, forming a dibromide

Question 14

What is Addition of Hydrogen Halides?

Answer 14

Alkenes react with either HBr, HCl, or HI to form alkyl halides

Question 15

HCl, HBr, and HI are unsymmetrical reagents that react with alkenes following this rule:

Answer 15

Markownikoff’s Rule

• Applies for unsymmetrical reagents being added to unsymmetrical multiple bonds
• States that Hydrogen from HX goes to the Carbon atom in double bond with the greater number of H atoms

Example: R-CH2CH=CH2 + HX -> R-CH2CHXCH3