Toluene is the simplest homolouge of benzene. It was first obtained by dry distillation of tolubalsam and hence named toluene. It is commercially known as tolual.
(1) Methods of preparation
(i) From benzene [Friedel-craft's reaction] :
Alkyl halide employed may undergo an isomeric change
\[AlC{{l}_{3}}\] are, \[AlC{{l}_{3}}>SbC{{l}_{3}}>SnC{{l}_{4}}>B{{F}_{3}}>ZnC{{l}_{2}}>HgC{{l}_{2}}\]
(ii) Wurtz fitting reaction :
(iii) Decarboxylation :
\[\underset{\text{Sodium}\,\text{toluate}}{\mathop{\underset{\text{(}o-,m-\,\text{or}\,p-)}{\mathop{{{C}_{6}}{{H}_{4}}\begin{matrix}C{{H}_{3}} \\\,\,\,\,\,\,COONa \\\end{matrix}}}\,}}\,+NaOH\xrightarrow{\text{Soda}\,\text{lime}}\underset{\text{Toluene}}{\mathop{{{C}_{6}}{{H}_{5}}C{{H}_{3}}}}\,+N{{a}_{2}}C{{O}_{3}}\]
(iv) From cresol :
(v) From toluene sulphonic acid :
(vi) From toluidine :
(vii) From grignard reagent :
(viii) Commercial preparation
From coal tar :
The main source of commercial production of toluene is the light oil fraction of coal-tar. The light oil fraction is washed with conc. \[{{H}_{2}}S{{O}_{4}}\] to remove the bases, then with \[NaOH\]to remove acidic substances and finally with water. It is subjected to fractional distillation. The vapours collected between \[80-110{}^\circ C\] is 90% benzol which contains \[70-80%\] benzene and \[14-24%\] toluene. 90% benzol is again distilled and the portion distilling between \[108-110{}^\circ C\] is collected as toluene.
(ix) From n- heptane and methyl cyclohexane
(2) Physical properties
(i) It is a colourless mobile liquid having characteristic aromatic odour.
(ii) It is lighter than water (sp. gr. 0.867 at \[{{20}^{o}}C\]).
(iii) It is insoluble in water but miscible with alcohol and ether in all proportions.
(iv) Its vapours are inflammable. It boils at \[{{110}^{o}}C\] and freezes at \[-{{96}^{o}}C\].
(v) It is a good solvent for many organic compounds.
(vi) It is a weak polar compound having dipole moment 0.4D.
(3) Chemical properties : Toluene shows the behaviour of both an alipatic and an aromatic compound.
(i) Electrophilic substitution reactions : Aromatic character (More reactive than benzene) due to electron releasing nature of methyl group.
\[{{E}^{+}}\] may be \[\overset{+}{\mathop{Cl}}\,,\overset{\,\,\,\,+}{\mathop{N{{O}_{2}}}}\,,\overset{\,\,\,\,\,\,\,\,\,\,\,\,\,+}{\mathop{S{{O}_{3}}H}}\,\] etc.
(ii) Reactions of side chain
(a) Side chain halogenation :
Benzyl chloride on hydrolysis with aqueous caustic soda forms benzyl alcohol.
(1) Directive effect in mono substituted benzene derivatives : The substituent already present on the benzene ring directs the incoming substituent to occupy ortho (2 or 6), meta (3 or 5) or para (4) position. This direction depends on the nature of the first substituent and is called directive or the orientation effect.
The substituent already present can increase or decrease the rate of further substitution, i.e., it either activates or deactivates the benzene ring towards further substitution. These effects are called activity effects.
There are two types of substituents which produce directive effect are,
(i) Those which direct the incoming group to ortho- and para-positions simultaneously (Neglecting meta all together).
(ii) Those which direct the incoming group to meta-position only (Neglecting ortho- and para-positions all together).
Theory of ortho - para directing group
The above mechanism is followed when S is \[-OH,\,-N{{H}_{2}},-Cl,\,-Br,-I,-OR,-N{{R}_{2}},-NHCOR\] etc.
In methyl or alkyl group, the +I effect of the methyl group or alkyl group initiates the resonance effect.
Thus, methyl or alkyl group directs all electrophiles to ortho and para positions.
Theory of meta directing group : The substituent, S withdraws electrons from ortho and para positions. Thus, m-position becomes a point of relatively high electron density and further substitution by electrophile occurs more...
Benzene is the first member of arenes. It was first discovered by Faraday (1825) from whale oil. Mitscherllich (1833) obtained it by distillating benzoic acid with lime. Hofmann (1845) obtained it from coal tar, which is still a commercial source of benzene.
(1) Structure of benzene : Benzene has a special structure, which is although unsaturated even then it generally behave as a saturated compound.
(i) Kekule's structure : According to Kekule, in benzene 6-carbon atoms placed at corner of hexagon and bonded with hydrogen and double bond present at alternate position.
(a) Evidence in favour of Kekule's structure
Benzene combines with 3 molecules of hydrogen or three molecules of chlorine. It also combines with 3 molecules of ozone to form triozonide. These reactions confirm the presence of three double bonds.
Studies on magnetic rotation and molecular refraction show the presence of three double bonds and a conjugated system.
The synthesis of benzene from three molecule of acetylene also favour's Kekule's structure.
Benzene gives cyclohexane by reduction with hydrogen.
(b) Objections against Kekule's formula
Unusual stability of benzene.
According to Kekule, two ortho disubstituted products are possible. But in practice only one ortho disubstituted product is known.
Heat of hydrogenation of benzene is 49.8 kcal/mole, whereas theoretical value of heat of hydrogenation of benzene is 85.8 kcal/mole. It means resonance energy is 36 kcal/mole.
\[C-C\] bond length in benzene are equal, (although it contains 3 double bonds and 3 single bonds) and are \[1.39\,\,\overset{o}{\mathop{A}}\,\].
Kekule explained this objection by proposing that double bonds in benzene ring were continuously oscillating between two adjacent positions.
(2) Methods of preparation of benzene
(i) Laboratory method :
(ii) From benzene derivatives
(a) From phenol :
(b) From chlorobenzene :
(c) By first preparing grignard reagent of chlorobenzene and then hydrolysed
\[\underset{\text{Chlorobenzene}}{\mathop{{{C}_{6}}{{H}_{5}}Cl}}\,\underset{\text{dry}\,\text{ether}}{\mathop{\xrightarrow{Mg}}}\,\underset{\text{chloride}}{\mathop{\underset{\text{Phenylmagnesium}}{\mathop{{{C}_{6}}{{H}_{5}}MgCl}}\,}}\,\xrightarrow{{{H}_{2}}O}\underset{\text{Benzene}}{\mathop{{{C}_{6}}{{H}_{6}}}}\,+Mg\ \ \ \ \begin{matrix}OH \\Cl \\\end{matrix}\]
(d) From benzene sulphonic acid :
(e) From benzene diazonium chloride :
(f) From acetylene :
Cyclic polymerisation takes place in this reaction.
(g) Aromatisation : \[\underset{n-Hexane}{\mathop{{{C}_{6}}{{H}_{14}}}}\,\underset{\text{at high pressure}}{\mathop{\underset{500{}^\circ C}{\mathop{\xrightarrow{C{{r}_{2}}{{O}_{3}}/A{{l}_{2}}{{O}_{3}}}}}\,}}\,\underset{\text{Benzene}}{\mathop{{{C}_{6}}{{H}_{6}}}}\,+4{{H}_{2}}\]
(3) Properties of benzene
(i) Physical properties
(a) Benzene is a colourless, mobile and volatile liquid. It's boiling point is \[{{80}^{o}}C\] and freezing point is \[{{5.5}^{o}}C\]. It has characteristic odour.
(b) It is highly inflammable and burns more...
(1) All arenes have general formula \[[{{C}_{n}}{{H}_{2n}}-6y]\]. Where y is number of benzene rings and n is not less than 6.
(2) Arenes are cyclic and planar. They undergo substitution rather than addition reactions.
(3) Aromaticity or aromatic character : The characteristic behaviour of aromatic compounds is called aromaticity. Aromaticity is due to extensive delocalisation of p-electrons in planar ring system. Huckel (1931) explained aromaticity on the basis of following rule.
Huckel rule : For aromaticity the molecule must be planar, cyclic system having delocalised \[(4n+2)\pi \]electrons where n is an integer equal to 0, 1, 2, 3, ………….
Thus, the aromatic compounds have delocalised electron cloud of 2,6,10 or 14 \[\pi \] electrons.
Similarly cyclolpentadienyl anion or tropylium ion are also aromatic because of containing \[6\pi \] electrons \[(n=1)\].
Hetrocyclic compounds also have 6p electrons (n = 1).
Molecules do not satisfy huckel rule are not aromatic.
(4) Antiaromaticity : Planar cyclic conjugated species, less stable than the corresponding acyclic unsaturated species are called antiaromatic. Molecular orbital calculations have shown that such compounds have \[4n\pi \] electrons. In fact such cyclic compounds which have \[4n\pi \] electrons are called antiaromatic compounds and this characteristic is called antiaromaticity.
Example : 1,3-Cyclobutadiene, It is extremely unstable antiaromatic compound because it has \[4n\pi \] electrons \[(n=1)\] and it is less stable than 1,3 butadiene by about 83.6 \[83.6\,\,KJ\,\,mo{{l}^{-1}}\].
Thus, cyclobutanediene shows two equivalent contributing structures and it has \[n=1\].
(1) Source of Arenes
Source of arenes is coal. It contains benzene, xylene, naphthalene etc. Arenes are obtained by destructive distillation of coal.
(2) Distillation of coal
Coal tar is a mixture of large numbers of arenes.
(3) Distillation of coal tar : Arenes are isolated by fractional distillation of coal tar,
These are hydrocarbon with two carbon-carbon double bonds. Dienes are of three types
(1) Conjugated dienes : Double bonds are seperated by one single bond.
Ex : \[C{{H}_{2}}=CH-CH=C{{H}_{2}}\] (1, 3-butadiene)
(2) Cumulative dienes : Double bonds are adjacent to each other.
Ex : \[C{{H}_{2}}=C=C{{H}_{2}}\] Propadiene [allene]
(3) Isolated or Non-conjugated : Double bonds are separated by more than one single bond.
Ex : \[C{{H}_{2}}=CH-C{{H}_{2}}-CH=C{{H}_{2}}\] (1, 4 pentadiene)
The general formula is \[{{C}_{n}}{{H}_{2n-2}}\]. The predominant member of this class is 1, 3-butadiene.
(1) Method of preparation
(i) From acetylene :
\[2HC\equiv CH\underset{N{{H}_{4}}Cl}{\mathop{\xrightarrow{C{{u}_{2}}C{{l}_{2}}}}}\,\underset{\text{Vinyl acetylene}}{\mathop{HC\equiv C-CH=C{{H}_{2}}}}\,\underset{Pd/BaS{{O}_{4}}}{\mathop{\xrightarrow{{{H}_{2}}}}}\,\] \[\underset{\text{1, 3-Butadiene}}{\mathop{C{{H}_{2}}=CH-CH=C{{H}_{2}}}}\,\]
(ii) From 1, 4-dichlorobutane :
\[\underset{\text{1,4-Dichlorobutane}}{\mathop{\overset{Cl\,\,\,}{\mathop{\overset{|\,\,\,\,\,}{\mathop{C{{H}_{2}}}}\,}}\,C{{H}_{2}}C{{H}_{2}}\overset{Cl\,\,\,}{\mathop{\overset{|\,\,\,\,\,}{\mathop{C{{H}_{2}}}}\,}}\,}}\,\xrightarrow{Alc.\,KOH}\underset{\text{1, 3-Butadiene}}{\mathop{C{{H}_{2}}=CH-CH=C{{H}_{2}}}}\,\]
(iii) From 1,4-butanediol :
\[\underset{\text{1, 4-Butanediol}}{\mathop{\overset{OH}{\mathop{\overset{|\,\,\,\,\,}{\mathop{C{{H}_{2}}}}\,}}\,C{{H}_{2}}C{{H}_{2}}\overset{OH}{\mathop{\overset{|\,\,\,\,\,}{\mathop{C{{H}_{2}}}}\,}}\,}}\,\underset{\text{heat}}{\mathop{\xrightarrow{{{H}_{2}}S{{O}_{4}}}}}\,\underset{\text{1, 3-Butadiene}}{\mathop{C{{H}_{2}}=CH-CH=C{{H}_{2}}}}\,\]
(iv) From butane :
\[\underset{\text{n-Butane}}{\mathop{C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}C{{H}_{3}}}}\,\underset{{{600}^{o}}C}{\mathop{\xrightarrow{\text{Catalyst}}}}\,\underset{\text{1, 3-Butadiene}}{\mathop{C{{H}_{2}}=CH-CH=C{{H}_{2}}}}\,\]
(\[C{{r}_{2}}{{O}_{3}}\] used as catalyst.)
(v) From cyclohexene :
(2) Physical property : 1,3-butadiene is a gas.
(3) Chemical properties
(i) Addition of halogens :
(ii) Addition of halogen acids :
(iii) Addition of water :
(iv) Polymerisation :
\[\underset{\text{1, 3-Butadiene}}{\mathop{nC{{H}_{2}}=CHCH=C{{H}_{2}}}}\,\xrightarrow{\text{Peroxide}}{{[-\underset{\text{Buna rubber}}{\mathop{C{{H}_{2}}CH=CHC{{H}_{2}}}}\,-]}_{n}}\]
Diels-alder reaction :
Stability of conjugated dienes : It is explained on the basis of delocalisation of electron cloud between carbon atoms.
The four \[\pi \] electrons of 1, 3-butadiene are delocalised over all the four atoms. This delocalisation of the \[\pi \] electrons makes the molecule more stable.
(v) Ozonolysis :
\[C{{H}_{2}}=CHCH=C{{H}_{2}}+2{{O}_{3}}\xrightarrow{Zn/{{H}_{2}}O}2HCHO+OHCCHO\]
Carbocyclic compounds with double bonds in the ring are called cycloalkenes. Some of the common cycloalkenes are
Cycloalkenes can be easily obtained by Diels-Alder reaction. These compounds undergo the electrophilic addition reactions which are characteristic of alkenes, while the ring remains intact. Cycloalkenes decolourise the purple colour of dilute cold \[KMn{{O}_{4}}\] or red colour of bromine in carbon tetrachloride.
Single bond between carbon atoms. Each carbon atom is \[s{{p}^{3}}-\]hybridized
Bond length \[1.54\,\overset{o}{\mathop{A}}\,\]
Bond energy :
\[83\text{ }Kcal\text{ }mo{{l}^{-1}}\]
Double bond between two carbon atoms. Both carbon atoms are \[s{{p}^{2}}-\]hybridized
\[1.34\overset{o}{\mathop{A}}\,\]
\[146\,\,Kcal\,\,mo{{l}^{-1}}\]
Triple bond between two carbon atoms both carbon atoms are sp-hybridized
\[-C\equiv C-\]
\[1.20\,\,\overset{o}{\mathop{A}}\,\]
\[200\,\,Kcal\,\,mo{{l}^{-1}}\]
Burning
Burns with nonluminous flame
\[{{C}_{2}}{{H}_{6}}+7/2{{O}_{2}}\to \]\[2C{{O}_{2}}+3{{H}_{2}}O\]
The gaseous mixture is passed through ammonical cuprous chloride solution. The alkyne (acetylene) reacts with \[C{{u}_{2}}C{{l}_{2}}\] and forms a red precipitate. It is filtered. The alkyne or acetylene is recovered by decomposition of the precipitate with an acid.
\[{{C}_{2}}{{H}_{2}}+C{{u}_{2}}C{{l}_{2}}+2N{{H}_{4}}OH\to \underset{(\text{Red ppt}\text{.)}}{\mathop{{{C}_{2}}C{{u}_{2}}}}\,+2N{{H}_{4}}Cl+2{{H}_{2}}O\]
\[{{C}_{2}}C{{u}_{2}}+2HN{{O}_{3}}\to {{C}_{2}}{{H}_{2}}+C{{u}_{2}}{{(N{{O}_{3}})}_{2}}\]
The remaining gaseous mixture is passed through concentrated \[{{H}_{2}}S{{O}_{4}}\]. Alkene is absorbed. The Hydrogen sulphate derivatives is heated at 170oC to regenerate ethene.
\[{{C}_{2}}{{H}_{4}}+{{H}_{2}}S{{O}_{4}}\xrightarrow{{}}{{C}_{2}}{{H}_{5}}HS{{O}_{4}}\underset{{{170}^{o}}C}{\mathop{\xrightarrow{\Delta }}}\,{{C}_{2}}{{H}_{4}}+{{H}_{2}}S{{O}_{4}}\]
The methane or ethane is left behind unreacted.
These are the acyclic hydrocarbons which contain carbon-carbon triple bond are called alkynes. General formula is \[{{C}_{n}}{{H}_{2n-2}}\]. Ex. Ethyne \[CH\equiv CH\]; Propyne \[C{{H}_{3}}-C\equiv CH\]
(1) General methods of preparation
In reaction with gem dihalide, Alc. KOH is not used for elimination in 2nd step.
In reaction with vicinal dihalide, if the reactant is 2-butylene chloride then product is 2-butyne as major product
Preparation of higher alkynes (by metal acetylide)
Acetylene gives salt with \[NaN{{H}_{2}}\] or \[AgN{{O}_{3}}\] (ammonical) which react with alkyl halide to give higher alkyne.
(2) Physical properties
(i) Acetylene is a colourless gas. It has a garlic odour. The odour is due to presence of impurities of phosphorous and hydrogen sulphide. However, pure acetylene has pleasant odour.
(ii) It is insoluble in water but highly soluble in acetone and alcohol. Acetylene is transported under high pressure in acetone soaked on porous material packed in steel cylinders.
(iii) Its boiling point is \[-{{84}^{o}}C\].
(iv) It is lighter than air. It is somewhat poisonous in nature.
(v) It burns with luminous flame and forms explosive mixture with air.
(3) Chemical reactivity of alkynes : \[C\equiv C\] is less reactive than the carbon-carbon double bond towards electrophilic addition reaction. This is because in alkyne carbon has more S-character so more strongly will be the attraction for \[\pi \] electrons. Alkyne also undergo nucleophilic addition with electron rich reagents. Ex. Addition of water, cyanide, carboxylic acid, alcohols. Nucleophilic addition can be explained on the basis that alkynes form vinylic carbanion which is more stable than alkyl carbanion formed by alkene
\[-C\equiv C-\,+N{{u}^{-}}\xrightarrow{{}}-\overset{Nu}{\mathop{\overset{|\,\,\,\,}{\mathop{C=}}\,}}\,\overset{}{\mathop{C}}\,-\]
Vinylic carbanion (more stable)
\[-C=C-\,\,+N{{u}^{-}}\xrightarrow{{}}\,\,\,-\overset{Nu}{\mathop{\overset{|\,\,\,\,}{\mathop{C-}}\,}}\,\overset{}{\mathop{C}}\,-\]
(alkyl carbanion) (less stable)
(i) Acidity of alkynes : Acetylene and other terminal alkynes (1- alkynes) are weakly acidic in character
Ex. \[CH\equiv CH+NaN{{H}_{2}}\xrightarrow{{}}H-C\equiv \overline{C}\,N{{a}^{+}}+\frac{1}{2}{{H}_{2}}\]
(Monosodium acetylide)
The acetylenic hydrogen of alkynes can be replaced by copper (I) and silver (I) ions. They react with ammonical solutions of cuprous chloride and silver nitrate to form the corresponding copper and silver alkynides.
\[CH\equiv CH+2[Cu{{(N{{H}_{3}})}_{2}}]Cl\xrightarrow{{}}Cu-C\equiv C-Cu+2N{{H}_{4}}Cl+2N{{H}_{3}}\]
Dicopper acetylide (Red ppt)
\[CH\equiv CH+2[Ag{{(N{{H}_{3}})}_{2}}]N{{O}_{3}}\xrightarrow{{}}AgC\equiv C-Ag+2N{{H}_{4}}N{{O}_{3}}+2N{{H}_{3}}\]
Disilver acetylide (white ppt)
This reaction can be used to distinguish between 2-alkynes and 1-alkynes. 1-alkynes will give this test while 2-alkynes, will not give this test.
\[\underset{\text{1-propyne}}{\mathop{C{{H}_{3}}-C\equiv CH}}\,+2[Ag{{(N{{H}_{3}})}_{2}}]\,N{{O}_{3}}\xrightarrow{{}}C{{H}_{3}}-C\equiv C-Ag\]
\[C{{H}_{3}}-C\equiv C-C{{H}_{3}}+2[Ag{{(N{{H}_{3}})}_{2}}]N{{O}_{3}}\xrightarrow{{}}\]No reaction
Explanation for the acidic character : It explained by \[sp\] hybridisation. We know that an electron in \[s-\]orbital is more tightly held than in a \[p\]-orbital. In \[sp\] hybridisation \[s\]-character is more (50%) as compared to \[s{{p}^{2}}\](33%) or \[s{{p}^{3}}\](25%), due to large \[s\]-character the carbon atom is quite electronegative.
(ii) Reaction with formaldehyde
\[\underset{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,OH}{\mathop{\underset{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,|}{\mathop{HC\equiv CH+2C{{H}_{2}}O\xrightarrow{{}}C{{H}_{2}}-C\equiv C}}\,}}\,-\underset{OH}{\mathop{\underset{|\,\,\,\,\,}{\mathop{C{{H}_{2}}}}\,}}\,\xrightarrow{Li/N{{H}_{3}}}C{{H}_{2}}-CH=CH-C{{H}_{2}}OH\]
\[\underset{OH\,\,\,\,\,}{\mathop{\underset{|\,\,\,\,\,\,\,\,\,\,\,\,}{\mathop{C{{H}_{2}}-}}\,}}\,CH=CH-C{{H}_{2}}OH\] [Trans-product ]
(4) Chemical properties of acetylene
Oxidative–Hydroboration : Alkynes react more...
\[AlC{{l}_{3}}>SbC{{l}_{3}}>SnC{{l}_{4}}>B{{F}_{3}}>ZnC{{l}_{2}}>HgC{{l}_{2}}\]
(ii) Wurtz fitting reaction :
(iii) Decarboxylation :
\[\underset{\text{Sodium}\,\text{toluate}}{\mathop{\underset{\text{(}o-,m-\,\text{or}\,p-)}{\mathop{{{C}_{6}}{{H}_{4}}\begin{matrix}C{{H}_{3}} \\\,\,\,\,\,\,COONa \\\end{matrix}}}\,}}\,+NaOH\xrightarrow{\text{Soda}\,\text{lime}}\underset{\text{Toluene}}{\mathop{{{C}_{6}}{{H}_{5}}C{{H}_{3}}}}\,+N{{a}_{2}}C{{O}_{3}}\]
(iv) From cresol :
(v) From toluene sulphonic acid :
(vi) From toluidine :
(vii) From grignard reagent :
(viii) Commercial preparation
From coal tar :
The main source of commercial production of toluene is the light oil fraction of coal-tar. The light oil fraction is washed with conc. \[{{H}_{2}}S{{O}_{4}}\] to remove the bases, then with \[NaOH\]to remove acidic substances and finally with water. It is subjected to fractional distillation. The vapours collected between \[80-110{}^\circ C\] is 90% benzol which contains \[70-80%\] benzene and \[14-24%\] toluene. 90% benzol is again distilled and the portion distilling between \[108-110{}^\circ C\] is collected as toluene.
(ix) From n- heptane and methyl cyclohexane
(2) Physical properties
(i) It is a colourless mobile liquid having characteristic aromatic odour.
(ii) It is lighter than water (sp. gr. 0.867 at \[{{20}^{o}}C\]).
(iii) It is insoluble in water but miscible with alcohol and ether in all proportions.
(iv) Its vapours are inflammable. It boils at \[{{110}^{o}}C\] and freezes at \[-{{96}^{o}}C\].
(v) It is a good solvent for many organic compounds.
(vi) It is a weak polar compound having dipole moment 0.4D.
(3) Chemical properties : Toluene shows the behaviour of both an alipatic and an aromatic compound.
(i) Electrophilic substitution reactions : Aromatic character (More reactive than benzene) due to electron releasing nature of methyl group.
The above mechanism is followed when S is \[-OH,\,-N{{H}_{2}},-Cl,\,-Br,-I,-OR,-N{{R}_{2}},-NHCOR\] etc.
In methyl or alkyl group, the +I effect of the methyl group or alkyl group initiates the resonance effect.
Thus, methyl or alkyl group directs all electrophiles to ortho and para positions.
Theory of meta directing group : The substituent, S withdraws electrons from ortho and para positions. Thus, m-position becomes a point of relatively high electron density and further substitution by electrophile occurs more... 
(b) Objections against Kekule's formula
(2) Methods of preparation of benzene
(i) Laboratory method :
(ii) From benzene derivatives
(a) From phenol :
(b) From chlorobenzene :
(c) By first preparing grignard reagent of chlorobenzene and then hydrolysed
\[\underset{\text{Chlorobenzene}}{\mathop{{{C}_{6}}{{H}_{5}}Cl}}\,\underset{\text{dry}\,\text{ether}}{\mathop{\xrightarrow{Mg}}}\,\underset{\text{chloride}}{\mathop{\underset{\text{Phenylmagnesium}}{\mathop{{{C}_{6}}{{H}_{5}}MgCl}}\,}}\,\xrightarrow{{{H}_{2}}O}\underset{\text{Benzene}}{\mathop{{{C}_{6}}{{H}_{6}}}}\,+Mg\ \ \ \ \begin{matrix}OH \\Cl \\\end{matrix}\]
(d) From benzene sulphonic acid :
(e) From benzene diazonium chloride :
(f) From acetylene :
Similarly cyclolpentadienyl anion or tropylium ion are also aromatic because of containing \[6\pi \] electrons \[(n=1)\].
Hetrocyclic compounds also have 6p electrons (n = 1).
Molecules do not satisfy huckel rule are not aromatic.
(4) Antiaromaticity : Planar cyclic conjugated species, less stable than the corresponding acyclic unsaturated species are called antiaromatic. Molecular orbital calculations have shown that such compounds have \[4n\pi \] electrons. In fact such cyclic compounds which have \[4n\pi \] electrons are called antiaromatic compounds and this characteristic is called antiaromaticity.
Example : 1,3-Cyclobutadiene, It is extremely unstable antiaromatic compound because it has \[4n\pi \] electrons \[(n=1)\] and it is less stable than 1,3 butadiene by about 83.6 \[83.6\,\,KJ\,\,mo{{l}^{-1}}\].
Thus, cyclobutanediene shows two equivalent contributing structures and it has \[n=1\].
(2) Physical property : 1,3-butadiene is a gas.
(3) Chemical properties
(i) Addition of halogens :
(ii) Addition of halogen acids :
(iii) Addition of water :
(iv) Polymerisation :
\[\underset{\text{1, 3-Butadiene}}{\mathop{nC{{H}_{2}}=CHCH=C{{H}_{2}}}}\,\xrightarrow{\text{Peroxide}}{{[-\underset{\text{Buna rubber}}{\mathop{C{{H}_{2}}CH=CHC{{H}_{2}}}}\,-]}_{n}}\]
Diels-alder reaction :
Stability of conjugated dienes : It is explained on the basis of delocalisation of electron cloud between carbon atoms.
The four \[\pi \] electrons of 1, 3-butadiene are delocalised over all the four atoms. This delocalisation of the \[\pi \] electrons makes the molecule more stable.
(v) Ozonolysis :
\[C{{H}_{2}}=CHCH=C{{H}_{2}}+2{{O}_{3}}\xrightarrow{Zn/{{H}_{2}}O}2HCHO+OHCCHO\]
Cycloalkenes can be easily obtained by Diels-Alder reaction. These compounds undergo the electrophilic addition reactions which are characteristic of alkenes, while the ring remains intact. Cycloalkenes decolourise the purple colour of dilute cold \[KMn{{O}_{4}}\] or red colour of bromine in carbon tetrachloride.
Bond energy :
\[83\text{ }Kcal\text{ }mo{{l}^{-1}}\]
\[1.34\overset{o}{\mathop{A}}\,\]
\[146\,\,Kcal\,\,mo{{l}^{-1}}\]
Oxidative–Hydroboration : Alkynes react
