CHAPTER 3 | ALKENE, ALKYNES AND ALKADIENES | ALKENE

B.Sc. 1st Year
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Organic Chemistry

## ALKENE

### Introduction :

Unsaturated hydrocarbons containing a carbon-carbon double bond in their molecules are called alkene‘. These are also called as olefins'(Latin; “olefiant” means oil forming ) since the lower members form oily products on reaction with halogens. Their general formula is CnH2n•

Alkenes are named by their common and I.U.P.A.C. name. In the common system, unsaturated hydrocarbons containing a C-C double bond are named by replacing the suffix -ane from the name of an alkane by -ylene, i.e. alkylene. For example : ethylene, propylene butylene etc. for olefins containing two, three, four carbons respectively.

### Nomenclature of Alkene :

In I.U.P.A.C. system, they are named as alkenes. The name of the individual alkenes is derived by replacing the suffix -ane of an alkane by -ene. For example : ethene, propene, butene etc. for alkenes containing two, three, four carbon atoms respectively. The I.U.P.A.C. naming of substituted alkenes are done according to the general rules of the I.U.P.A.C. system of nomenclature. For example :

### Preparation of Alkene :

Alkenes can be prepared by the following methods. 1. Dehydrohalogenation of Alkyl halides Removal of a molecule of hydrogen halide(HCI, HBr or HI) from the adjacent carbon atoms of an alkyl halide is known as Vehydrohalogenation’. By heating an alkyl halide with a strong base such as sodium ethoxide or a conc. alcoholic solution of KOH. For example :

1. ### Dehalogenation of vicinal dihalides :

The di-halogen compounds in which both halogen atoms —rx,… attached to the adjacent carbon-atoms are called vicinal dihalides’. When heated with zinc a-a-1h as:ohol, these vicinal dihalides lose a molecule of halogefra-s-Zific -halide to form alkenes. This reaction is as “dehalogenation”.

### 3. Dehydration of alcohols:

The dehydration(removal of a molecule of water) from an alcohol is carried by heating the alcohol with protonic acids such as conc. H2SO4 or H3PO4 Lewis acids such as 1:—.7drous zinc chloride or alumina can also be used.

### propan-1 -ol Hofmann Elimination :

Pyrolysis of quarternary ammonium salts results in the formation of alkenes. This is called the “Hofmann elimination reaction”.

1. ### Reduction of Alkynes :

Controlled reduction of alkynes with hydrogen in the presence of Palladised ::um carbonate in quinoline to produce alkenes.

### Physical propeties :

The first three alkenes(ethene, propene and butene) are gases at ordinary temperature ; the next fourteen members are liquids and the alkenes containing more than eighteen carbon atoms are solids. They dissolve freely in organic solvents. Their boiling points, melting points and specific gravities, in general rise with increase of molecular weight in the homologous series.

### Chemical propeties:

Alkenes contain a weak n-bond between carbon atoms apart from C-C, G-bond. Their it- electrons are less tightly held and can attract electrophiles and repel nucleophiles.

Alkenes, therefore, easily undergo electrophilic addition reactions across their double bonds. Electrophilic additions are addition reactions initiated by the attack of an electrophile.

As a result of electrophilic addition, a weak TE – bond (251 KJ mol-l) is broken and two strong G – bonds (2 x 347 = 694 KJ mol-l) are formed. There is a net gain of 443KJ mol-1 of energy (i.e. 694 – 251). Thus,addition reactions are energitically favourable ones as energy is released in comparison with electrophilic substitution reactions. Some important electrophilic addition reactions alongwith their mechanisms are briefly discussed in this chapter.

Halogens such as Cl2 and Br2 readily add to alkenes to form 1, 2 – dihaloalkanes. For example :

The order of reactivity of halogens to alkenes is:

Fluorine > Chlorine > Bromine > Iodine

Fluorine reacts with alkenes too rapidly and exothermally to be controlled while iodine does not react with alkenes under ordinary conditions. During the addition of bromine to alkenes, the orange red colour of bromine is disappeared since the dibromoalkane is colourless. This reaction is, therefore, used as a test for unsaturation in organic compound especially.

### Mechanism :

To illustrate this mechanism, let us take the addition of bromine to ethylene as an example. the mechanism consists of two steps ;

Step 1 : The π- electrons of ethylene molecule attacks the vacant orbital of one of the bromine atom to form a π- complex. The electron – electron repulsion cause the polarisation of Br – Br. bond enabling 7_7e 7t- electrons to attack the electron deficient bromine atom. The π- complex subsequently forms a carbocation and a bromide anion. This slow step is the rate- determining step of the reaction

However. the formation of a carbocation fails to explain the following :

(i)The absence of product, cis – 1,2 – dibromocycloalkane in the addition of bromine to cycloalkenes.

(ii)The lack of rearranged products as carbocation can undergo rearrangement.

### Cyclo Halonium ion Mechanism :

Since the addition of halogens to alkenes always give trans – :.Aides and the rearrangement of intermediate carbocation is not obserbed, it is therefore postulated that -e mechanism involves a cyclic bromonium ion instead of a simple carbocation. The cyclic bromonium ion probably  formed a π- complex.

In cyclic bromonium ion, the attack of the bromide ion can occur only from the backsides of the bromine m (forming the bridge) since the attack from the front side will be hindered by this bulky bromine atom. is is why addition of halogen to alkene gives trans – 1,2- dihaloalkenes.

1. ### Addition of halogen acids :

Halogen acids like HCI, HBr or HI add to alkenes forming alkyl halides. Y 10- –C-C-1 , Alkyl halide.

The order of reactivity of halogen acid in this reaction:

HI           >HBr         >HCI

It is accordance with the bond dissociation energy of halogen acid.

Halogen acid:                                    HI                                  HBr                            HCl

Dissociation energy(KJ):                  300                               360                           430

The least bond dissociation energy of HI makes it the most reactive.

### Mechanism :

The addition of halogen acids to alkenes is also an electrophilic addition reaction that involves the following steps :

Addition of peroxide to alkene changes the order of the addition of the reagent to the double bond making the ring appear to violate Markonikov’s Rule while alkenes are symmetrical, only one product is expected tc form. On the other hand, addition of halogen acids to unsymmetrical alkenes, two products are possible with one being the major product.

For example: addition of HCI on propene in the dark and in the absence of peroxides give two products; the major product is 2 – chloropropane and minor one is 1 – chloropropane.

### Markonikov’s Rule :

Markonikov, a Russian Chemist postulated this rule in order to give explanation to the major product. The rule states that when an unsymmetrical reagent adds to an unsymmetrical alkene, the positive part of the addendum (i.e. adding molecule) get attached to that double bonded carbon atom which carries greater number of hydrogen atoms. For example :

### Explanation :

In electrophilic addition reactions, the first step involves the addition of a proton. r: can occur in two . :f the proton adds on the terminal carbon atom (C1) of the double bond, a secondary (2′) carbocation is formed. secondary carbocation is more stable (due to greater number of hyperconjugating forms ) than the primary (1′) –.7.:_,:cation the predominant product will be accordingly 2 – bromopropane.

Thus, Markonikov’s aOdition occurs through rcre stable carbocation intermediate. 7.Y.-:xicie Effect- Anti -Markonikov’s addition: Addition of HBr (not HCi or HI) to unsymmetrical alkenes in of peroxide [ benzoyl peroxide; (C6H5COO)2 ] occurs contrary to Markonikov’s rule. This effect is also as the ‘Kharasch effect’.

### Mechanism :

The addition of HBr to unsymmetrical alkene in presence of peroxide occurs by a free radical mechanism The oxide is used to generate free radicals. It consists of three steps.

Step I Initiation

Why only HBr shows peroxide effect ?

The paroxide effect is observed only with HBr because both the substeps of propagation are exothermic. On hand with HF, HCI or HI one of the substeps is endothermic and therefore can not be occurred.

#### L Addition of Sulphuric acid :

When cold conc. H2SO4 is added to alkenes, alkyl hydrogen sulphates we ‘:—.ed. When the alkene is unsymmetrical, addition occurs in accordance with Markonikov’s rule. For example

### Mechanism :

It involves the following steps-

### Application :

Alkyl hydrogen sulphates when boiled with water undergoes hydrolysis to produce alcohols For example:

Thus, alkenes can be hydrated indirectly through alkyl hydrogen sulphates.

1. #### Addition of Water/Direct hydration (Formation of Alcohols) :

Water adds only to reactive alkene: in presence of mineral acids to form alcohols. In case of unsymmetrical alkenes, addition occurs accordance with with Markonikov’s rule. For examples :

### Mechanism :

In this electrophilic addition, in the first step the it — electrons attack the proton; H+ to form th: carbocation which combines with water to form alcohol (with the elimination of Fl+ ion).

Step l: Electrophilic attacks of proton; H+ ion on alkene.

Step II : Attacks of a water molecule (nucleophile) on tert – butyl carbocation.

1. Hydroboration – Indirect hydration of Alkenes (Formation of Alcohols) :

Diborane adds to alkenes 😮 form trialkyl boranes. In this reaction all of the three hydrogen atoms are replaced by alkyl groups followed by successive additions. For example : Propene on treatment with diborane.

This reaction is called ‘hydroboration’. Here 1-1- ion is the nucleophile and Here B- icn :s the = follows Markonikov’s rule.

### Mechanism :

It involves stereospecifically cis- addition, in accordance with the anti- • .’s rule :

Note :

1. B2H6 is a dimeric form of boron trihydride; BH3.
2. Boron trihydride ; BH3.is an electron deficient, Lewis acid and very reactive molecule.

iii. Herbert Charles Brown won Nobel Prize in 1979 with a Wittig for their studies in boron containing organic cQmp-ounds.

#### Explanation :

The anti – Markonikov’s direction of addition to form the 1° trialkylborane may be explained by the fact that due to electron-deficient nature, borane(BH3) molecule behaves as electophile and thus it attacks at the point of highest electron density.

### Application of Hydroboration :

The importance of this reaction is its well-known application. When trialkyl boranes are oxidised with an alkaline solution of hydrogen peroxide, alcohols are obtained.

The overall process is known as ‘hydroboration – oxidation’. The significance of this process is the formation of 1° alcohols from alkenes, contrary to Markonikov’s rule.

The other significance of this reaction is that when trialkyl-borane is treated with propanoic acid, the corresponding alkane is obtained by protolysis :

. Oxidation Reactions of Alkene :

Alkenes are readily oxidised by different oxidizing agents forming different- products. -Some of the .important oxidation reactions are discussed below :

### (i) Epoxidation :

Alkenes when treated with peracids such as perbenzoic acid; C6H5C000H, peroxy trifluoroacetic acid (CF3C000H) etc in an inert solvent such as chloroform or carbon tetrachloride epoxides (oxiranes) are obtained. This reaction is known as epoxidation’. In general.

Emmons et al (1954, 1955) have found that CF3C000H is very good reagent for epoxidation and hydroxylation.

### Mechanism :

Many mechanisms have been proposed for epoxidation, but none is certain. According to Pausacker et al (1955), the mechanism involves the following steps.

Ethylene oxide is one of the most important epoxides and may also be obtained by oxidation with the silver catalyst at 250°C.

(ii) Hydroxylation :

Alkenes when treated with cold dilute aqueous alkaline solution of KMnO4, glycols are formed.Glycols are those compounds which contain two hydroxyl groups on adjacent carbons. The reaction looks like the addition of two hydroxyl groups to a double bond. In general,

1. ### Ozonolysis :

When ozone is bubbled through the solution of an alkene in an inert solvent like CHCI3, 3CI4, etc at low temperature (195-200 K ), form an unstable intermediate called’ monozonide11,2,4- trioxolane) which subsequently rearranges to form more stable ozonide(1,2,4 trioxolane). This reaction involving the addtion of ozone to alkene to form ozonide is called ozonization’. For example :

Ozonides are unstable explosive compounds. Therefore, they are never isolated but can be converted into carbonyl compounds by reductive cleavage. When the ozonides are treated with reducing agent such as – dust and H2/ or Zn in acetic acid / or H2 and Pd, it undergo cleavage and forms carbonyl compounds depending upon the structure of the alkene.

This two step process involving the formation of ozonide and its reductive clevage to yield carbonyl compounds is called ozonolysis. However, if the ozonide when treated in the absence of zinc, the H202 croduced during the reaction, oxidise the carbonyl compounds( aldehydes) to corresponding acids. This -eaction is called ‘oxidative cleavage’.

### Application :

Ozonolysis is a useful method for locating the position of double bond in an unknown alkene by studying the products obtained after reductive cleavage. Therefore, this method has been used for the structure elucidation of olefins. For examples :

If more than one double bonds are present, different set of more carbonyl accordingly. For example :

Thus, ozonolysis is also used as a means of preparing different aldehydes and ketones.

Note

(i) Ozonolysis is a complete process of preparing the ozonide and its decomposition.

(ii) The ozonide is prepared by dissolving the olefinic compound (alkene) in a solvent that is affected by 03, e.g. CHCI3, CCI4, glacial CH3COOH, light petrol, etc.

(iii) The ozonide is oxidized by means of silver oxide, hydrogen peroxide, or peracids thereby producing acids and/or ketones.

(iv) Reduction of the ozonide with Zn-HCI, H2 /Raney Ni, triphenyl phosphine etc gives aldehydes and / or ketones. However, the reduction of ozonide with LiAIH4 or NaBH4 gives the products of corresponding alcohols of the carbonyl compounds.