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Preparation of Alkadienes & its properties

Chapter

General formula of alkadienes is CnH2n2

Preparation of alkadienes

(i) Dehydrogenation of butane

(ii) Pyrolysis of Cyclo alkenes

This reaction is known as Retro Diels Alder reaction.

(iii) Dehydration of diols

(iv) Dehydrohalogenation by alc. KOH

(v) From HCHO

Properties of alkadienes

(a) In 1,3- Butadiene all carbons are in sp2 hybridisation.

(b) 1,3- Butadiene undergo resonance so that it undergo electrophilic addition reaction at both 1,2-and 1,4- position of molecule.

(c) In Butadiene C2 C3 bond length is 1.48 which is less than C C bond length of ethane while C1C2 or C3 C4 bond length

is 1.37 which is slightly more than C = C of ethene.

(d) 1,3 - Butadiene undergo following type of reaction

(i) Electrophilic addition reaction (ii) Free radical addition reaction

(iii) Polymerisation (iv) Cyclic reaction

Electrophilic addition reaction

[i] Electrophilic addition reaction

1,3- Butadiene undergo electrophilic addition reaction at both 1,2-and 1,4-position due to resonance.

At low temperature mainly forms product of 1,2-position while at high temperature 1,4-product is the major one.

Different product 1,3-butadiene at different temperature based on concept of kinetics versus thermodynamical control.

Hydrogen halides add to 1,3-butadiene to give a mixture of two isomeric allylic halides. For the case of ionic addition of hydrogen

bromide,

When the major product formation by addition of a proton at C-1 and bromide at C-2 in 1,3-butadiene this type of addition is called

1,2 addition, or direct addition. The minor product has its proton and bromide at C-1 and C-4, respectively, of the original diene

system. This addition is called 1,4 addition, or conjugate addition. The double bond that was between C-3 and C-4 in the starting

material remains there in the product from 1,2 addition but migrates to a position between C-2 and C-3 in the product from 1,4

addition.

Both the 1,2-addition and the 1,4-addition are take place from the same allylic carbocation as intermediate.

The secondary carbon atom contains more positive charge than the primary carbon, and attack by the nucleophilic bromide ion is

faster there. Hence, the major product is the secondary halide.

When the major product of a reaction is the one that is formed at the fastest rate, we say that the reaction is control by kinetic

control. While hydrogen bromide reacts with 1,3-butadiene at low temperature is a kinetically controlled reaction.

The ionic addition of hydrogen bromide to 1,3-butadiene is carried out at room temperature, the ratio of isomeric allylic bromides

observed is different from those which are formed at low temperature. At room temperature, the 1,4-addition product gives the

major part of the reaction product as thermodynamical product.

The temperature at which reaction occurs, exerts a major influence on the product composition. The 1,2- and 1,4-addition products

interchange rapidly by allylic rearrangement at elevated temperature in the presence of hydrogen bromide. Heating the product

mixture to 40C in the presence of hydrogen bromide give a mixture where the ratio of 3-bromo-1-butene to 1-bromo-2-butene is

20 : 80.

The product of 1,4 addition, 1-bromo-2-butene, contains an internal double bond and so is more stable than the product of 1,2

addition, 3-bromo-1-butene, which has a terminal double bond. To explain the concept of kinetics versus thermodynamic products,

here we take addition of HCN to butenone. Two products can be formed.

Alkadienes and Cycloalkanes

Alkadienes and Cycloalkanes

Chapter

The direct addition to the left means that cyanide ion must attack the carbonyl group directly while the conjugate addition to the

right means that it must attack the less electrophilic alkene. The second is a slower reaction but gives more stable product, point to

remember.

(i) The thermodynamic product has a lower energy than the kinetic product.

(ii) Initially the reaction will go to the left, if there is enough energy for the kinetic product to get back to the starting materials,

there will be enough energy for some thermodynamic product to be formed.

(iii) The kinetic product is formed reversibly ; the thermodynamic product irreversibly.

(iv) The energy needed for the thermodynamic product to get back to starting materials is very great.

(v) At low temperatures direct addition is favoured, but conjugate addition is favoured at high temperature for example 1,3-

butadiene form 1-butene at low temperature by hydrogenation while at high temperature form more stable 2-butene.

We take some other example also to understand this concept, for example

(a)

The two alkenes are labelled E and Z. After about 2 hours the main product is the Z-alkene. However, this is not the case in the

early stages of the reaction. The graph below shows how reaction proceeds.

(i) When the alkyne concentration drops almost to zero (10 minutes), the only alkene that has been formed is the E-alkene.

(ii) As the time increases, the amount of E-alkene decreases as the amount of the Z-alkene increases.

(iii) Eventually, the proportions of E and Z-alkenes do not change.

If we examine, these above reactions easily explain concepts of kinetics and thermodynamical product. After about 2 hour the main

product is benzaldehydeoxime because it is more thermally stable than cyclohexanoneoxime. In early states due to more kinetics of

cyclohexanone it forms major product but as the time goes and temperature also increases we prefer thermodynamical product.

When 1,3-butadiene undergo hydrogenation at room temperature by one mole of H

2

it forms 2-butene as major product. Reactions of this type are said to be governed by

thermodynamic control.

At low temperature, addition takes place irreversibly. Isomerization is slow because insufficient thermal energy is available to

permit the products to surmount the energy barrier for ionization but at higher temperatures isomerization is possible, so more

stable product predominates.

Free radical addition reaction

[ii] Free radical addition reaction

Mechanism

[iii] Cyclic reaction

(a) Electrocyclic reaction By the effect of temperature when conjugated polyene forms cyclic isomerised product known as

electrocyclic reaction.

These are reversible chemical reaction.

(b) Cyclo addition reaction When conjugated diene system reacts with some 2p electron system it means alkene, alkyne or its

derivative to form six membered cyclic compound. This reaction is known as Diels - alder reaction.

[iv] The Diels-Alder reaction

The Diels-Alder reaction is the conjugate addition of an alkene to a diene, Using 1,3-butadiene as a typical diene, the Diels-

Alder reaction may be represented by the general equation following general reaction where polarity occurs and followed by

addition.

The alkene that adds to the diene is called the dienophile. Because the Diels-Alder reaction leads to the formation of a ring, it is

termed a cycloaddition reaction. The product contain a cyclohexene ring as a structural unit.

The Diels-Alder cycloaddition is one example of a class called pericyclic reactions. A pericyclic reaction is a one-step reaction

that proceeds through a cyclic transition state. Bond formation occurs at both ends of the diene system, and the Diels. Alder

transition state involves a cycle formation of six carbons and six p electrons. The diene must adopt the cis conformation in the

transition state.

The most common and simplest of all Diels-alder reactions, cycloaddition of ethene to 1, 3-butadiene, does not proceed readily. It

has a high activation energy and a low rate of reaction. However, substituents such as C = O or C N, when directly attached to the

double bond of the dienophile, increase its reactivity, and compounds of this type give high yields of Diels-Alder products at very

normal temperature.

The product of a Diels-Alder cycloaddition always contains one more ring than already present in the reactants. The dienophile

maleic anhydride contains one ring, so the product of its addition to a diene contains two

Alkadienes and Cycloalkanes

Acetylene, like ethylene, is a poor dienophile, but alkynes that bear C == O or C N as side chain react readily with dienes. A

cyclohexadiene derivative is the product.

The Diels-Alder reaction is stereospecific in nature means substituents that are cis in the dienophile remain cis in the product while

substituents that are trans in the dienophile remain trans in the product.

Condition for Diels alder reaction

(a) Reaction occurs between conjugated diene (enophile) and some p electron system (dienophile).

(b) Reaction always occurs at 1,4-position of conjugated diene where six member cyclic compound formation takes place.

(c) Reaction does not occur when conjugated system contains bulky alkyl group due to steric hinderance.

(d) Reaction occurs only in cisoid system not in transoid due to more distance.

Diels - Alder reaction of benzyne

Diels - Alder reaction of benzyne

Alternative methods for generation of benzyne have made it possible to work as an intermediate in a number of synthetic

applications. One such method involves treating o-Iodofluorobenzene with magnesium, normally in tetrahydrofuran as the solvent.

The reaction proceeds by formation of the Grignard reagent from o-Iodofluorobenzene. Since the order of reactivity of magnesium

with aryl halides is ArI > ArBr > ArCl > ArF, the Grignard reagent has the structure shown below and forms benzyne by loss of

the salt FMgI:

Its strained triple bond makes benzyne a relatively good dienophile, and when benzyne is generated in the presence of a conju gated

diene, Diels-Alder cycloaddition occurs.

If two moles of benzyne reacts with each other they form Biphenylene.

[v] Ozonolysis of alkadiene

Preparation of cycloalkanes

(i) By Perkins Method

(ii) By pyrolysis of divalent metal salts of dicarboxylic acids (Wislicenus method)

(iii) By Freund reaction

(iv) By the use of carbenes

Carbenes are unstable intermediates in which carbon has pair of unshared electrons. These are neutral species and have no for mal

charge, :CH2(methylene) is the simplest member of this series. On account of greater stability di halo carbenes (:CX2) are also

used. Carbenes are easily prepared in solution as shown below and in the same form they are used for the reaction. This method is

very useful for the preparation of small ring compounds.

Methylene is prepared as follows

Cyclopropane and its derivatives can easily be prepared by the use of carbenes.

Reduction of a cyclic ketone

(v) Reduction of a cyclic ketone

The Wolff-Kishner carbonyl reduction is a good method for converting carbonyl group directly to methylene group. It involves

heating the hydrazone of carbonyl compound in the presence of an alkali and a metal catalyst.

The Clemmensens and Wolff- Kishner carbonyl reductions complement each other, for the former is carried out in acid solution,

and the latter in alkaline solution. Thus, the Clemmensen carbonyl reduction may be used for alkali-sensitive compounds where as

Wolff-Kishner carbonyl reduction is the method of choice with acid sensitive compounds.

Properties of cycloalkanes

The cycloalkanes resemble the alkanes in physical properties. They are insoluble in water but soluble in many organic solvents and

are lighter than water. The melting and boiling points of cycloalkanes are ten to twenty degrees higher than the corresponding

alkanes (open chain molecules). But for cyclopropane and cyclobutane, cycloalkanes are relatively inert towards the action of

common reagents at room temperature. These two compounds are exceptions and show a tendency to react with opening of the

ring. Cyclopropane forms addition products with ring fission, as shown :

Thus cyclopropane and to a lesser extent, cyclobutane reacts by addition; all other cycloalkanes show the reactivity expected by

alkanes, i.e. they react by substitution. In diffused light, cyclopropane reacts with chlorine to give substitution product.

Stability of cycloalkanes

Cyclopropane and cyclobutane resemble alkenes to a certain extent, because these compounds undergo addition reactions.

The higher cycloalkanes do not undergo addition reactions but give substitution reactions. Cyclopropane and its derivatives a re

most reactive and least stable where as, cyclobutane is less reactive and cyclopentane is still less reactive more stable.

Adolf Van Baeyer, a German chemist, in 1885, proposed a theory to explain the relative stability of cycloalkanes. The theory is

based on the fundamental concept of regular tetrahedral structure as given by Vant Hoff and Le Bel. According to them the four

valencies of carbon atom are directed towards the four corners of a regular tetrahedron with carbon atom at the centre making an

angle of 109o28 between any pair of such valencies.

According to Baeyers strain theory the valence angles can be altered from the normal value of 109o28 and when this alternation

is done, an internal strain is setup in the molecule. The greater the deviation from the normal angle, greater is the strain and

consequently lesser is the stability of the molecule. The main assumption in the theory is that all the carbon atoms forming the ring

must lie in the same plane. Thus the degree of stability of ring is inversely proportional to the amount of deviation from the normal

angle of 109o28. Bayer proposed that this deviation could be calculated as shown :

= (10928 bond angle) where all the carbon atoms in the ring are in the same plane. Suppose there are

n

carbon atoms in the ring, then the bond angle of ring would be

Most of the compounds tends to convert in stable ring like we know that cyclopropane is

maximum strain ring so that it want to convert in cyclopentane and cyclohexane derivatives

in different chemical reaction for example.

In all above example due to formation of intermediate carbocation, expansion of ring can

takes place.

It can be seen from the above data that the lowest value of the devitation is found in five and

six-membered rings and so cyclopentane and cyclohexane are the most stable systems.

The negative value of d indicates that the bonds are bent outwards. The Baeyers strain theory could easily explain the greater reactivity of cyclopropane and cyclobutane and the

stability of cyclopentane and cyclohexane rings. The strain theory agrees reasonably with

properties of alicyclics containing six or less number of carbon atoms it is now clear that why

only the 1,4 and 1,5 dicarboxylic acids form cyclic anhydrides whereas 1,6 and 1,7 form

cyclic ketones etc.

Degree of Unsaturation (DU)

The number of pairs of H atom a molecular formula lacks to be an alkane is called the degree

or element of unsaturation (DU). The number of elements of unsaturation.

Where n1 is number of carbon atom while n2 is number of hydrogen atom.

# 1 degree of unsaturation C == C bond or one ring

# 2 degree of unsaturation means C == C, or one C == C or two ring or one ring and one C

== C bond.

# Halogen is like H, O is not counted in oxygen containing compounds and one H is counted

less in N containing compounds.

b) C8H12 elements of unsaturation DU = 3

You can also make many other arrangements

# C3H3Cl3 (It is like C3H6) = 1 unsaturation

# C3H4O (It is like C3H4) = 2 unsaturation

# C4H5N (It is like C4H4) = 3 unsaturation

For example, this molecule contains four degree of unsaturation because

cyclopropyl ring is there which undergo addition reaction.

In above example cyclohexane is stable ring, so it cannot react in dark but in the presence of

sunlight free radical substitution reaction occurs.