HYDROCARBONS, THE FIRST FAMILY

Experiment 6

PREPARATION OF ACETYLENE (ETHYNE)

Hydrocarbons -- contains only C and H atomsClassifications:1. Alkanes

- C-C sigma and C-H sigma bonds; sp3 hybridized

- undergo reactions involving free radicals

2. Alkenes

- Csp2-Csp

2 orbital overlap

- undergo electrophilic addition

3. Alkynes

- 2 pi bonds and 1 sigma bond

- undergoes electrophilic addition

4. Aromatic Hydrocarbons

- presence of pi system of electrons

Types of Reactions:

1.) Alkanes:

Halogenation

- free radical substitution

- Br2/CCl4

-positive test: loss of red-orange color of molecular bromine and the evolution of gas

2.) Alkenes:

Addition reactions

- electrophilic addition

a. ) Br2/CCl4 (does not require the presence of light)

- positive test: loss of the red-orange color of molecular bromine and the evolution of gas

b.) Baeyer’s reagent (cold, dilute neutral aqueous KMnO4)

- positive test: loss of purple color and the formation of a brown precipitate (MnO2)

3.) Alkynes

- electrophilic addition

Halogenation

- 2 Br2/CCl4 (does not require the presence of light)

- positive test: loss of red-orange molecular bromine and evolution of gas

Oxidation

- Baeyer’s reagent (cold, dilute, neutral aqueous KMnO4)

- positive test: loss of purple color; formation of brown ppt

Ammoniacal AgNO3

- test for terminal alkynes

- release of terminal proton of the alkyne

- positive test: insoluble Ag+ ppt (acetylide ion)

Solubility TestsA. Investigation of Hydrocarbons1. Physical Properties

Cyclohexane

- liquid

less

colorless

CyclohexeneBenzene

-

- liquid

lessless

colorless

colorless

Solubility in water

Physical state

liquid

Density in water

color

2. Halogenation: Reaction with bromine

In the dark Light

Cyclohexane

Red-orange Colorless

Cyclohexene

Colorless Colorless

Benzene Red-orange

Red-orange

Reactions Involved:

Br2/CH2Cl2

N.R.dark

Br2/CH2Cl2light

Br+ HBr

Br2/CH2Cl2light or dark

BrBr + H2

Br2/CH2Cl2light or

dark

N.R

H-C = C-HBr2/CCl4

light or dark

H- C = C - H

Br Br

Br2/CCl4;light or dark

H - C - C - H Br Br

Br Br

3. Reaction with Baeyer’s rgt and Ammoniacal AgNO3

Sample KMnO4 (cold,dilute.neutral) Ag(NH3)2+

Cyclohexane - -Cyclohexene + -

Benzene - -

Reactions Involved:

KMnO4

H2ON.R

KMnO4

H2OOH

OH+ MnO2 + KOH

KMnO4, H2O

N.R

H - C = C - HKMnO4, H2O

H3C-C-OH + MnO2 +KOH

O

Ag(NH3)2

+H-C=C-Ag+ +NH3+NH4

+

4. Preparation of Acetylene gas

CaC2 + 2H2O C2H2 (g) + Ca(OH)2

RELATIVE BROMINATION RATES OF BENZENE AND SUBSTITUTED BENZENES

EXERCISE 7

E

H

E

H

E

H

E+

E

H

GE

H

GE

H

GE+

G

MECHANISM OF ELECTROPHILIC AROMATIC SUBSTITUTION

With a substituent group G

Substituted rings are divided into two groups based on the type of the substituent that the ring carries:

1. Activated rings: the substituent on the ring are groups that donate electrons (activators). 

-NH2, -NR2 > -OH, -OR>  -NHCOR> -CH3 and other alkyl groups

2. Deactivated rings: the substituent on the ring are groups that withdraw electrons(deactivators).

-NO2, -CF3 > -COR, -CN, -CO2R, -SO3H > Halogens

Halogens: F> Cl > Br > I

ACTIVITY:

1.Observe the relative bromination rate of substituted benzene

2.Compare bromination rates of the given compounds

3.Make a relative classification of the substituent group as ring activating or deactivating based on experimental observations