Industrial methods for alkene synthesis
On an industrial scale, unsaturated compounds are synthesized from alkanes. The main feature of these processes is the use of high temperatures to break chemical bonds.
Dehydrogenation is the reaction of removing a hydrogen molecule from an alkane. The process requires both intense heating and the presence of a specific catalyst.
- Classic example: heating butane to 450 °C in the presence of chromium(III) oxide ($Cr_2O_3$) results in the abstraction of hydrogen atoms. This reaction predominantly yields but-2-ene and releases free hydrogen gas.
- Reaction equation: $CH_3-CH_2-CH_2-CH_3 \xrightarrow[450^\circ C]{Cr_2O_3} CH_3-CH=CH-CH_3 + H_2$
Cracking is the thermal decomposition of large alkane molecules into smaller fragments. For example, during the thermal cleavage of a long-chain molecule (such as $C_{12}H_{26}$), alkenes are inevitably formed in the product mixture. This is one of the fundamental processes in petroleum refining.
Laboratory methods: elimination reactions
In laboratory practice, the synthesis of alkenes is carried out under significantly milder conditions. The basis is formed by elimination reactions, in which various halogen derivatives of alkanes serve as starting materials.
Dehydrohalogenation is the joint elimination of a halogen atom and a hydrogen atom.
- Reagents: chloro- or bromoalkanes treated with strong alkalis (sodium hydroxide $NaOH$ or potassium hydroxide $KOH$) are used for the reaction.
- Mandatory condition: the reaction proceeds exclusively in an alcoholic solution. Using an aqueous solution will lead to the formation of an alcohol rather than an alkene.
- Mechanism: halogen and hydrogen atoms are eliminated from two strictly adjacent carbon atoms. The freed valences close to form a new double bond, while water and a salt are released.
- Example: the action of an alcoholic alkali solution on 1-chloropropane yields propene.
$Cl-CH_2-CH_2-CH_3 + NaOH \xrightarrow{alcohol} CH_2=CH-CH_3 + NaCl + H_2O$
Regioselectivity and Zaitsev's rule
When working with asymmetric haloalkanes, the question arises: from which specific adjacent carbon atom will hydrogen depart? In such cases, chemists are guided by Zaitsev's rule.
> Rule formulation: In elimination reactions, a hydrogen atom is predominantly abstracted from the less hydrogenated carbon atom (i.e., the one initially bonded to a smaller number of hydrogen atoms).
Let us examine this principle using the reaction of 2-chlorobutane with an alcoholic alkali solution. The chlorine atom is located at the second carbon atom. The adjacent carbon atoms are $C^1$ ($CH_3$ group, three hydrogen atoms) and $C^3$ ($CH_2$ group, two hydrogen atoms). Since atom $C^3$ has fewer hydrogens, elimination occurs specifically from it. As a result, the double bond forms between the second and third carbon atoms, and but-2-ene (rather than but-1-ene) becomes the main reaction product. $CH_3-CH_2-CHCl-CH_3 + NaOH \xrightarrow{alcohol} CH_3-CH=CH-CH_3 + NaCl + H_2O$
Dehalogenation of dihaloalkanes
Another effective laboratory method is dehalogenation.
- Reagents: dihaloalkanes in which two halogen atoms are attached to adjacent carbon atoms are taken as starting materials. Active metals, most commonly zinc ($Zn$) or magnesium ($Mg$), are added to them.
- Mechanism: the active metal abstracts and binds both halogen atoms, converting into a salt (metal halide). The remaining free electrons at the adjacent carbon atoms form a carbon-carbon double bond.
- Example: the reaction of 1,2-dibromobutane with metallic zinc yields but-1-ene and zinc bromide.
$CH_2(Br)-CH(Br)-CH_2-CH_3 + Zn \rightarrow CH_2=CH-CH_2-CH_3 + ZnBr_2$
Briefly: answers to topical questions
What are the temperature conditions and products of alkane cracking?
Alkane cracking occurs at high temperatures and leads to the formation of an alkane and an alkene.
Process features:
- Essence of the reaction is the thermal decomposition (cleavage) of the initial hydrocarbon molecule into two parts.
- Cleavage site — the carbon chain breaks predominantly in the middle of the molecule.
- Catalysts — aluminosilicates are used in the reactions.
Cleavage example: the alkane molecule $C_{16}H_{34}$ yields the alkane $C_8H_{18}$ and the alkene $C_8H_{16}$.
To easily remember Zaitsev's rule, use the principle "the poor get poorer": in elimination reactions, hydrogen is taken from the carbon atom that already had fewer hydrogens to begin with.
Class and exam questions
In what medium should dehydrohalogenation take place to produce an alkene?
The reaction must proceed strictly in an alcoholic alkali solution (NaOH or KOH).
What is the main product formed during the dehydrogenation of butane?
Heating in the presence of a catalyst predominantly forms but-2-ene and releases a hydrogen molecule.
Why are zinc or magnesium used in dehalogenation reactions?
These active metals bind the halogen atoms located on adjacent carbons, leading to the formation of a double bond and an alkene.
What else to study on this topic
- Mechanisms of elimination reactions in organic chemistry
- Differences in reaction products of haloalkanes in aqueous and alcoholic media
- Influence of catalyst structure on the dehydrogenation process
- Application of Zaitsev's rule to branched hydrocarbons
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