Chemistry > Hydrocarbons > 14.0 Chemical Properteis
Hydrocarbons
1.0 Introduction
2.0 Alkanes
3.0 Methods of Preparation Alkanes
4.0 Physical Proparties
5.0 Chemical Properties
6.0 Alkenes
7.0 Methods of Preparation Alkenes
7.1 Dehydrohalgoenation
7.2 Dehydration of Alcohols
7.3 Dehalogenation
7.4 Thermal elimination reaction
7.5 By partial reduction of alkynes:
7.6 Wittig Reaction
7.7 Kolbe hydrocarbon synthesis
8.0 Physical Proparties
9.0 Chemical Properties
10.0 Mechanism Of Some Important Reaction Of Alkenes
10.1 Mechanism of halogen addition
10.2 Mechanism of halohydrin formation
10.3 Syn - hydroxylation
10.4 Oxidation reactions of alkenes
11.0 Alkynes
12.0 Methods of Preparation Alkynes
12.1 Industrial source
12.2 Kolbe’s method
12.3 Dehydrohalogenation of 1, 2 – dihalides
12.4 Dehydrohalogenation of 1, 1 – dihalides
12.5 Dehalogenation of tetrahalides or trihalides
12.6 Alkylation of acetylene and terminal alkynes
13.0 Physical Properties
14.0 Chemical Properteis
14.1 Electrophilic addition reactions
14.2 Acidity of Alkynes
14.3 Aromatic Hydrocarbons
14.4 Structure of Benzene
15.0 Modern Concept
15.1 Aromaticity in Benzene and Related Systems
15.2 Huckel’s rule or $\left( {{\bf{4n}}{\text{ }} + {\text{ }}{\bf{2}}} \right)\pi $ electron rule
16.0 Properteis
17.0 Mechanism of Electrophilic Substitution Reactions
17.1 Nitration
17.2 Friedel – Craft Alkylation
17.3 Friedel – Craft Acylation
17.4 Reactions of side chains
18.0 Toluene
19.0 Alkenyl Benzene
14.1 Electrophilic addition reactions
7.2 Dehydration of Alcohols
7.3 Dehalogenation
7.4 Thermal elimination reaction
7.5 By partial reduction of alkynes:
7.6 Wittig Reaction
7.7 Kolbe hydrocarbon synthesis
10.2 Mechanism of halohydrin formation
10.3 Syn - hydroxylation
10.4 Oxidation reactions of alkenes
12.2 Kolbe’s method
12.3 Dehydrohalogenation of 1, 2 – dihalides
12.4 Dehydrohalogenation of 1, 1 – dihalides
12.5 Dehalogenation of tetrahalides or trihalides
12.6 Alkylation of acetylene and terminal alkynes
14.2 Acidity of Alkynes
14.3 Aromatic Hydrocarbons
14.4 Structure of Benzene
15.2 Huckel’s rule or $\left( {{\bf{4n}}{\text{ }} + {\text{ }}{\bf{2}}} \right)\pi $ electron rule
17.2 Friedel – Craft Alkylation
17.3 Friedel – Craft Acylation
17.4 Reactions of side chains
Alkynes undergo electrophilic addition reactions but less readily because of the following reasons:
(a) C – atoms of alkynes being sp – hybridized are more electronegative and hence hold the $\pi - electrons$ more tightly.
(b) $\pi - electrons$ of alkynes are more delocalized and hence are less easily available for reaction.
(c) Vinyl carbocation formed after the addition of an electrophile to the alkyne is far less stable than the alkyl carbocations obtained by the addition of an electrophile to the alkene.
Some of the electrophilic addition reactions of alkynes are discussed below.
(i) Addition of halogens:
First trans – dihalides are formed which react further to form tetrahalides.
The order of reactivity with halogens is
$$C{l_2} > B{r_2} > {I_2}$$
(ii) Addition of Hydrogen
\[{C_2}{H_2}\xrightarrow[{Catalyst}]{{{H_2}}}{C_2}{H_4}\xrightarrow[{Catalyst}]{{{H_2}}}{C_2}{H_6}\]
If the triple bond is not present at the end of chain of the molecule then the dialkyl acetylene may be catalytically reduced to cis and trans alkanes.
(iii) Addition of halogen acids
The addition is in accordance of markovnikov’s rule.
\[CH \equiv CH\xrightarrow[{}]{{HBr}}C{H_2} = CHBr\xrightarrow[{}]{{HBr}}C{H_3}CHB{r_2}\]
Because of - I effect of the bromine atom, the availability of the $\pi $-electrons in the first step is decreased and the addition is much slower as compared to ethylene. The reactivity is $HI{\text{ }} > {\text{ }}HBr > {\text{ }}HCl.$. When passed through dilute $HCl$ in the presence of $H{g^{2 + }}$ as catalyst, acetylene forms vinyl chloride.
(iv) Addition of water
when passed into dilute ${H_2}S{O_4}$at ${60^0}C$ in the presence of $HgS{O_4}$as catalyst acetylene adds on one molecule of water to form acetaldehyde.
The homologous of acetylene forms ketone when hydrated.
(v) Reaction with metals
A triply bonded carbon which is sp – hybridized has a high electronegativity and hydrogen atoms attached to it show appreciable acidity.
$e.g.$
$CH{C^ - }N{a^ + }$ is also obtained by passing acetylene into a solution of sodium in liquid ammonia until the blue colour disappears.
Sodium acetylide can be used to prepare higher members of alkyne series.
\[CH \equiv {C^ - }N{a^ + }\xrightarrow{{RX}}CH \equiv CR + NaX\]
When acetylene is passed into an ammonical solution of $C{u_{2}}C{l_2} or {\text{ }}AgN{O_3},$ cuprous acetylide $C{u_2}{C_2}$ (red) or silver acetylide $A{g_2}{C_2}$(white) is precipitated.
\[CH \equiv CH + \underbrace {2AgN{O_3} + 2N{H_4}OH}_{Tollen's\,\,reagent}\xrightarrow{{}}A{g^ + }{C^ - } \equiv {C^ - }A{g^ + } + 2N{H_4}N{O_3} + 2{H_2}O\]
\[CH \equiv CH + C{u_2}C{l_2}\xrightarrow{{N{H_4}OH}}C{u^ + }{C^ - } \equiv {C^ - }C{u^ + } + 2N{H_4}Cl + 2{H_2}O\]
(vi) Ozonolysis
\[\mathop {C{H_3}C \equiv C - C{H_3}}\limits_{Butyne - 2} \xrightarrow[{{H_2}{O_2}}]{{{O_3}/{H_2}O}}2C{H_3}COOH\]
Acetylene is exceptional that it gives glyoxal as well as formic acid.
(vii) Polymerisation
When passed through a heated tube (Fe, Cu or Ni), acetylene changes to small extent of benzene.
1, 3 , 5 – trimethyl benzene (mesitylene)
When passed into a solution of $C{u_2}C{l_2}in{\text{ }}N{H_4}Cl,$ acetylene gives vinyl acetylene.
\[\mathop {CH \equiv CH + CH \equiv CH}\limits_{} \xrightarrow[{N{H_4}Cl}]{{C{u_2}C{l_2}}}C{H_2} = CH - C \equiv CH\]
But – 3 – en – 1 – yne
Compounds containing both double and triple bonds are named as alkenynes
(viii) Isomerization
When alkynes are heated with NaNH2 in an inert solvent triple bond moves towards the end of chain.
