Hydrocarbons

Preparation Of Alkanes

A. Reactions with No Change In Carbon Skeleton


1. Reduction of Alkyl Halides (RX, X = F, Cl, Br or I)

(Substitution of halogen by hydrogen)

a) RX + Zn: + H⁺ RH + Zn²⁺ + X^-

b) 4RX + LiAlH₄ 4RH + LiX + AlX₃ (XF)

or RX + H:^(-) RH + X^- (H^- comes from LiAlH₄)

c) RX + (n - C₄H₉)₃ SnH R-H + (n - C₄H₉)₃ SnX


d) via organometallic compounds (Grignard Reagent). Alkyl halides react with either Mg or Li in dry ether to give organometallics having a basic carbanionic site.





The net effect is replacement of X by H.







2. Preparation of Alkanes with More c's Than the Starting Compounds

Two R groups can be coupled by reacting RBr, RCl or RI with Na or K, yields of product are best for 1^o (60%) and least for 3^o (10%) alkyl halides (Wurtz Reaction).

2RX + 2Na R-R + 2NaX
2Na + 2CH₃CH₂CH₂Cl CH₃CH₂CH₂CH₂-CH₂CH₃ + 2NaCl

A superior method for coupling is the Corey-House Synthesis.

R MgX or RLi R-R' (R = 1^o, 2^o or 3^o; R' = 1^o)

3. By heating a mixture of the sodium salt of a carboxylic acid and soda-lime
R CO₂Na + NaOH (CaO) RH + Na₂CO₃

This process of eliminating CO₂ from a carboxylic acid is known as decarboxylation.

4. Kolbe's electrolytic method : A concentrated solution of the sodium or potassium salt of a carboxylic acid is electrolysed.
2 RCOOK + 2H₂O R-R + 2CO₂ + H₂ + 2KOH



Chemical Properties
Halogenation

• Chlorination may be brought about by photo irradiation, heat or catalysts, and the extent of chlorination depends largely on the amount of chlorine used. A mixture of all possible isomeric monochlorides is obtained, but the isomers are formed in unequal amounts, due to difference in reactivity of primary, secondary and tertiary hydrogen atoms.
• The order of ease of substitution is

Tertiary Hydrogen > Secondary Hydrogen > Primary Hydrogen

• Iodination is reversible, but it may be carried out in the presence of an oxidising agent such as HIO₃, HNO₃ etc., which destroys the hydrogen iodide as it is formed and so drives the reaction to the right,

e.g.
CH₄ + I₂CH₃I + HI

5HI + HIO₃ 3I₂ + 3H₂O

• Iodides are more conveniently prepared by treating the chloro or bromo derivative with sodium iodide in methanol or acetone solution. e.g

RCl + NaI RI + NaCl

• This reaction is possible because sodium iodide is soluble in methanol or acetone, whereas sodium chloride and sodium bromide are not. This reaction is known as Conant Finkelstein reaction.
• In more complex alkanes, the abstraction of each different kind of H atom gives a different isomeric product. Three factors determine the relative yields of isomeric product.

1. Probability Factor: This factor is based on the number of each kind of H atom in the molecule. For example, in CH₃CH₂CH₂CH₃ there are six equivalent 1^o H's and four equivalent 2^o H's. The odds on abstracting a 1^oH are thus 6 to 4, or 3 to 2.

2. Reactivity of The order of reactivity of H is 3^o > 2^o > 1^o.

3. Reactivity of The more reactive is less selective and more influenced by the probability factor. The less reactive width="20" height="16"> is more selective and less influenced by the probability factor, as summarized by the Reactivity-Selectivity Principle. If the attacking species is more reactive, it will be less selective, and the yields will be closer to those expected from the probability factor.

• In the chlorination of isobutane abstraction of one of the nine primary hydrogens leads to the formation of isobutyl chlorides, whereas abstraction of a single tertiary hydrogen leads to the formation of tert-butyl chloride. The probability favour formation of isobutyl chloride by the ratio of 9:1. But the experimental results show the ratio roughly to be 2:1 or 9:4.5. Evidently, about 4.5 times as many collisions with the tertiary hydrogen are successful as collisions with the primary hydrogens. The E_act is less for abstraction of a tertiary hydrogen than for abstraction of a primary hydrogen.
• The rate of abstraction of hydrogen atoms is always found to follow the sequence 3^o > 2^o > 1^o. At room temperature, for example, the relative rate per hydrogen atom are 5.0:3.8:1.0. Using these values we can predict quite well the ratio of isomeric chlorination products from a given alkane.
• The same sequence of reactivity, 3^o > 2^o > 1^o, is found in bromination, but with enormously larger reactivity ratios. At 127^oC, for example, the relative rates per hydrogen atom are 1600:82:1. Here, differences in reactivity are so marked as vastly to outweigh probability factors. Hence bromination gives selective product.

Preparation Of Alkenes

Cracking of petroleum hydrocarbons is the source of commercial alkenes.


a) (Mainly a special industrial process)

Most alkenes are made in the laboratory by b-elimination reactions.


b) (Dehydrohalogenation)


KOH in ethanol is most often used as the source of the base, , which then is C₂H₅O^-.
c)
d)

• In dehydration and dehydrohalogenation the preferential order for removal of an H is 3°>2°>1° (Saytzeff Rule). We can say "the poor gets poorer." That is because the more R's on the C = C group, the more stable is the alkene. The stability of alkenes in decreasing order of substitution by R is,
  R₂C = CR₂ > R₂C = CRH > R₂C = CH₂ ; RCH = CHR > RCH = CH₂ > CH₂ = CH₂

e) Alkynes can also be partially reduced to alkenes.



Dehydrohalogenation belongs to a general class of reaction: 1,2-elimination.


Such elimination reactions are characterized by the following:

a) The substrate contains a leaving group, an atom or group that leaves the molecule, taking its electron pair with it.

b) In a positionto the leaving group, the substrate contains an atom or group - nearly always hydrogen- that can be abstracted by a base, leaving its electron pair behind.

c) Reaction is brought about by action of a base.

• Typically, the base is a strongly basic anion like hydroxide, or an alkoxide derived from an alcohol : ethoxide, C₂H₅O^-., tert-butoxide, (CH₃)₃CO^-; etc.
• In elimination, a good leaving group is a weakly basic anion or molecule, just as in nucleophilic substitution. As a weak base, it readily releases a proton; as a good leaving group, it readily releases carbon. In dehydrohalogenation the leaving group is the very weakly basic halide ion. Other substrates which can release weakly basic anions are sulfonates.

Heterolytic bond dissociation energies show that strength of carbon-halogen bonds follow the sequence,

Heterolytic bond dissociation energy R-F > R-Cl > R-Br > R-I

An Exception to Saytzeff's RuleCarrying out dehydrohalogenations with a base such as potassium tert-butoxide in tert-butyl alcohol favours the formation of the less substituted alkene :



When an elimination yields the less substituted alkene, we say that, it follows the Hofmann rule.

Dehydration of Alcohols

Heating most alcohols with a strong acid causes them to lose a molecule of water (to dehydrate) and form an alkene.



Dehydration reactions of alcohols show several important characteristics which shall be explained.

1. The experimental conditions-temperature and acid concentration-that are required to bring about dehydration are closely related to the structure of the individual alcohol.

Ease of Dehydration
3° Alcohol > 2° Alcohol > 1° Alcohol

This behaviour, is related to the stability of the carbocation formed in each reaction.
2. Some primary and secondary alcohols also undergo rearrangements of their carbon skeleton during dehydration. Rearrangements of carbocations can also lead to the change in ring size, as the following example shows:



Chemical Properties of Alkenes

• Alkenes undergo addition reactions at the double bond. The p electrons of alkenes are a nucleophilic site and they react with electrophiles by three mechanisms.

Alkenes in the presence of organic peroxides reacts with hydrogen bromide, undergoes anti-Markovnikov addition. Hydrogen flouride, hydrogen chloride, and hydrogen iodide do not give anti-Markovnikov addition even when peroxides are present. The mechanism for anti-Markovnikov addition of HBr is a free radical chain reaction initiated by peroxides:


Addition of Water to Alkenes: Acid Catalyzed Hydration

The acid-catalyzed addition of water to the double bond of an alkene is a method of preparation of low molecular weight alcohols. The addition of water to the double bond follows Markovnikov's rule.



As the reactions follow M.R, acid-catalyzed hydration of alkenes do not yield primary alcohols except in the special case of the hydration of ethene. The occurance of carbocation rearrangements limits the utility of alkene hydrations as a laboratory method for preparing alcohols. Oxymercuration-demercuration, allows addition of H and OH without rearrangements. Another called hydroboration-oxidation, permits the anti-Markovnikov and syn-addition of H and OH.


Oxymercuration-Demercuration

Alkenes react with mercuric acetate in the presence of water to give hydroxymercurial compounds which on reduction yield alcohols.



The first stage, oxymercuration, involves addition to the C-C double bond of -OH and HgOAc. Then, in demercuration, HgOAc is replaced by H. The reaction sequence amounts to hydration of the alkene, but is much more widely applicable than direct hydration.

Hydroboration-Oxidation

With the reagent diborane, (BH₃)₂, alkenes undergo hydroboration to yield alkylboranes, R₃B, which on oxidation gives alcohols. For example :




The hydroboration oxidation process gives products corresponding to anti-Markovnikov addition of water to the C-C double bond, however it is Markovnikoff addition in real sense. This reaction is free from rearrangement.

Addition Of Bromine And Chlorine to Alkenes

Alkenes react rapidly with bromine at room temperature and in absence of light. If bromine is added to an alkene, the red-brown colour of the bromine disappears almost instantly as long as the alkene is present in excess. The reaction is one of addition .





Halohydrin Formation
If the halogenation of an alkene is carried out in aqueous solution (rather than in CCl₄), the major product of the overall reaction is a halo-alcohol called a halohydrin. In this case, the molecules of the solvent becomes reactants, too.




Halohydrin formation can be explained by the following mechanism :




Syn-Hydroxylation

Hydroxylation with permanganate is carried out by reaction at room temperature of the alkene and aqueous permanganate solution ; either neutral or slightly alkaline. Hydroxylation is a very good method for the synthesis of 1,2-diols.
The mechanism for the formation of glycols by permanganate ion and osmium tetroxide involves the formation of cyclic intermediates. Then in several steps cleavage at the oxygen-metal bond takes place ultimately producing the glycol and MnO₂ or Os metal. The course of these reactions is syn-hydroxylation.

Oxidative Cleavage of Alkenes

Alkenes are oxidatively cleaved by hot alkaline permanganete solution. The terminal CH₂ group of 1-alkene is completely oxidized to CO₂ and water. A disubstituted atom of a double bond becomes align="absmiddle"> group of a ketone.

• A monosubstituted atom of a double bond become aldehyde group which is further oxidised to salts of carboxylic acids.

Ozonolysis of Alkenes

A more widely used method for locating the double bond of an alkene involves the use of ozone (O₃). Ozone reacts vigorously with alkenes to form unstable compounds called initial ozonides, which rearranges spontaneously to form compounds known as ozonides. Ozonides, themselves are unstable and reduced directly with Zn and water. The reduction produces carbonyl compounds that can be isolated and identified