Distinguishing Hydrocarbons

7/23/2019 Distinguishing Hydrocarbons

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EXPERIMENT 1

Reactions of Hydrocarbons

Aim

- To investigate the polarity of hydrocarbons

- To distinguish between different types of hydrocarbons – alkanes, alkenes, benzenes- To observe bromination and permanganate oxidation reactions of alkanes, alkenes,

benzenes

Introduction

Hydrocarbons are compounds which contain only carbon and hydrogen atoms. liphatic

hydrocarbons are divided into three classes! alkanes have only single bonds, and are said to be

saturated" alkenes and alkynes have carbon-carbon double or triple bonds, and are said to be

unsaturated.

Alkanes have the general formula #nH$n%$, where n is a natural number &',$,(,)*. The simplestalkane is methane gas, #H+. The primary source of alkanes is petroleum. lkanes are relatively

inert to chemical oxidizing agents such as neutral or alkaline permanganate, where alkenes are

readily oxidized at room temperature. The change in color can be used as a test for a double

bond, provided the molecule contains no easily oxidizable group. lkanes are almost totallyinsoluble in water because of their low polarity and their disability to form hydrogen bonds.

iuid alkanes are soluble in one another, and they are generally dissolved in solvents of low

polarity such as benzene #H, #arbon tetrachloride ##l+, chloroform #H#l( and other organiccompounds. lkanes react slowly or not at all with bromine at room temperature in the dark, but

in the presence of sunlight, substitution is fairly rapid!

/-H % 0r $ 1light /-0r % H0r &a substitution reaction*

2$ 1light

2

% & H0r *

The reaction is easily detected by loss of the bromine color and by evolution of hydrogen

bromide.

Alkenes and cycloalkenes are hydrocarbons which contains a carbon–carbon double bond!

C C

The simplest alkene is ethene &ethylene* gas. The general formula of lkenes is #nH$n and of cycloalkenes is #nH$n-$. The simplest cycloalkenes have physical properties similar to those of

corresponding lkenes. lkenes are very reactive with compared to alkanes because of the

double bond, which can be opened.



C C C C

3hile typical reactions of alkanes are substitution reactions &such as with halogens*, typical

reactions of alkenes are addition reactions to the double bond.

ddition of hydrogen halides to alkenes. 4or example!

C C

H

CH3

CH3

H

H Cl C CH

CH3

CH3

HCl

H

+

0romine adds rapidly at room temperature to alkenes in a reaction, which does not reuire light.5ince the product is colorless, the bromine is rapidly decolorized when added to an alkene.

The reagent is otentially dangerous! "ee it off your skin and clothes! Protect

your eyes!

# 6 #2$ 1 #H$#l$

#- #

2

2

Procedure

'. 7lace $ m of each substance to separate test tubes. dd ( m of water to each of the test

tubes, and cork it with a stopper. 5hake it gently and carefully remove the stopper. 3atch

to see if the hydrocarbons are soluble in water. /epeat with benzene. 8escribe your

observations.

$. 0romine Test - add ' m of each hydrocarbon to a small test tube, hydrocarbon to ( - +

m of $9 bromine in carbon tetrachloride &0r $1##l+*. 5hake well and observe after two

or three minutes. :f the bromine is not decolorized prepare a second similar tube and

place on tube in your laboratory locker and the other in bright sunlight. llow both tubes

to stand for ten to fifteen minutes and compare them. ;bserve the color of each tube, and

whether or not hydrogen bromide was evolved and record the results.

(. dd $ m of the substance in $ m of water. dd $9 potassium permanganate solution

dropwise and observe the result.

Treatment of data

• /eport your results in tabular form under the headings!

T<5T ;05</=T:;> #;>#?5:;>



• 8escribe two tests each on how to prepare an alkene, alkyne and alkane, with euations.

• 8raw skeletal &line – bond* structures for the following compounds! cyclohexane and

cyclohexene.

• 3hat are the general formulae for alkanes and cycloalkanes@

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Distinguishing Hydrocarbons