Alkenes: production methods, chemical properties and application

In organic chemistry, one can find hydrocarbon substances with different amounts of carbon in the chain and a C = C bond. They are homologues and are called alkenes. Because of their structure, they are chemically more active than alkanes. But what kind of reactions are typical for them? Consider their distribution in nature, different methods of preparation and application.

What are they like?

Alkenes, which are also called olefins (oily), got their name from ethene chloride, a derivative of the first representative of this group. All alkenes have at least one double C = C bond. C _n H _2n is the formula of all olefins, and the name is formed from an alkane with the same number of carbons in the molecule, only the suffix -an changes to -ene. The Arabic numeral at the end of the name with a hyphen denotes the carbon number from which the double bond begins. Consider the basic alkenes, the table will help you remember them:

If the molecules have a simple unbranched structure, then add the suffix -ilene, this is also shown in the table.

Where can I meet them?

Since the reactivity of alkenes is very high, their representatives in nature are extremely rare. The principle of life of the olefin molecule is “let's be friends”. There are no other substances around - it does not matter, we will be friends with each other, forming polymers.

But they are, and a small number of representatives are part of the associated petroleum gas, and the highest - in the oil produced in Canada.

The very first representative of the alkenes ethene is a hormone that stimulates the ripening of fruits, so representatives of the flora synthesize it in small quantities. There is an alkene cis-9-tricosen, which in female houseflies plays the role of a sexual attractant. It is also called muskalur. (Attractant - substances of natural or synthetic origin, which causes attraction to a source of smell in another organism). From the point of view of chemistry, this alkene looks like this:

Since all alkenes are a very valuable raw material, the methods for producing them artificially are very diverse. Consider the most common.

And if you need a lot?

In industry, the class of alkenes is mainly obtained by cracking, i.e. cleavage of a molecule under the influence of high temperatures, higher alkanes. The reaction requires heating in the range of 400 to 700 ° C. The alkane breaks down as it wants, forming alkenes, the methods for which we are considering, with a large number of molecular structures:

C ₇ H ₁₆ -> CH ₃ -CH = CH ₂ + C ₄ H _10.

Another common method is called dehydrogenation, in which a hydrogen molecule is separated from a representative of a series of alkanes in the presence of a catalyst.

In laboratory conditions, alkenes and methods of preparation are different; they are based on elimination reactions (splitting of a group of atoms without their substitution). Most often, water atoms from alcohols, halogens, hydrogen or hydrogen halide are eliminated. The most common method for producing alkenes is from alcohols in the presence of an acid, as a catalyst. Other catalysts are also possible.

All elimination reactions are subject to the Zaitsev rule, which states:

The hydrogen atom is split off from the carbon adjacent to the carbon carrying the —OH group, which has less hydrogen.

When applying the rule, answer which reaction product will prevail? Later you will find out if you answered correctly.

Chemical properties

Alkenes actively react with substances, breaking their pi-bond (another name for the bond is C = C). After all, it is not as strong as a single (sigma-bond). Hydrocarbon from unsaturated is converted to saturated, without forming other substances after the reaction (addition).

The following are the most common alkene reactions that occur in different human activities:

• addition of hydrogen (hydrogenation). The presence of catalyst and heating is necessary for its passage;
• addition of halogen molecules (halogenation). It is one of the qualitative reactions to the pi-bond. Indeed, during the reaction of alkenes with bromine water, it becomes transparent from brown;
• reaction with hydrogen halides (hydrohalogenation);
• water addition (hydration). The reaction conditions are heating and the presence of a catalyst (acid);

The reactions of asymmetric olefins with hydrogen halides and water obey the Markovnikov rule. So, hydrogen will join that carbon from a double carbon-carbon bond, which already has more hydrogen atoms.

• combustion;
• incomplete oxidation is catalytic. The product is cyclic oxides;
• Wagner reaction (permanganate oxidation in a neutral environment). This alkene reaction is another qualitative C = C bond. When a pink solution of potassium permanganate flows, it discolors. If the same reaction is carried out in a combined acidic environment, the products will be different (carboxylic acids, ketones, carbon dioxide);
• isomerization. All types are characteristic: cis and trans, double bond displacement, cyclization, skeletal isomerization;
• polymerization is the main property of olefins for industry.

Medical use

Of great practical importance are the reaction products of alkenes. Many of them are used in medicine. Glycerin is obtained from propene. This polyhydric alcohol is an excellent solvent, and if used instead of water, the solutions will be more concentrated. For medical purposes, alkaloids, thymol, iodine, bromine and others are dissolved in it. Glycerin is also used in the preparation of ointments, pastes and creams. It prevents them from drying out. Glycerin itself is an antiseptic.

When reacting with hydrogen chloride, derivatives are obtained that are used as local anesthesia when applied to the skin, as well as for short-term anesthesia with minor surgical interventions, using inhalation.

Alkadienes are alkenes with two double bonds in one molecule. Their main application is the production of synthetic rubber, from which they then make various heating pads and syringes, probes and catheters, gloves, nipples, and much more, which is simply irreplaceable when caring for patients.

Industrial application

Alkenes and their derivatives are widely used in industry. (Where and how alkenes are used, table above).

This is only a small part of the use of alkenes and their derivatives. Every year the need for olefins only increases, which means that the need for their production also increases.

(Answer: butene-2.)