organic chemistry: alkene notes

7/29/2019 Organic Chemistry: Alkene Notes

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ALKENESAlkenes: Hydration (Direct Addition of Water)

The addition of water to an alkene in the presence of a catalytic amount of strong acid

leads to the formation of alcohols (hydroxy-alkanes).

This reaction proceeds via a standard carbocation mechanism and follows the

Markovnikov rule. The mechanism for the addition of water to ethene follows.

1. The hydrogen ion is attracted to the bond, which breaks to form a bond withone of the double-bonded carbons. The second carbon of the original double-

bonded carbons becomes a carbocation.

2. An acid-base reaction occurs between the water molecule and the carbocation,forming an oxonium ion.

3. The oxonium ion stabilizes by losing a hydrogen ion, with the resulting formationof an alcohol.


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Alkenes: Halogenation

Halogenation is the addition of halogen atoms to a -bond system. For example, the

addition of bromine to ethene produces the substituted alkane 1,2-dibromoethane.

The reaction proceeds via a trans addition, but because of the free rotation possible

around the single bond of the resulting alkane, a trans product cannot be isolated. If,

however, the original alkene structure possesses restricted rotation due to a factor

other than a double bond, a trans-addition product can be isolated. For instance, ringstructures possess restricted rotation. In a ring structure, the carbon backbone is

arranged so there is no beginning or ending carbon atom. If cyclohexene, a six-carbon

ring that has one double bond, is halogenated, the resulting cycloalkane

is trans substituted.

Mechanism and stereochemistry of halogenation. Alkenes and halogens arenonpolar molecules. However, both types of molecules, under proper conditions, can

undergo induced-dipole formation, which leads to the generation of forces of attraction

between the molecules.

The bromoethyl carbocation that forms mid reaction in this example is often internally

stabilized by cyclization into a three-membered ring containing a positively charged

bromine atom (bromonium ion).


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This intermediate is more stable than the corresponding linear carbocation because all

the atoms have a complete octet of electrons.

The bromonium ion shares the electrons in the carbon-bromine covalent bond

unevenly, with the overlap region being closer to the more electronegative bromine.

This generates a partial positive charge (+) on the carbon atoms of the ring. The

charge delocalization stabilizes the ring structure, and the resulting partial positive

charges on the carbon atoms attract the nucleophilic bromide ion.

The second bromide ion must approach a partially positive carbon atom from the side of

the carbocation opposite where the bromonium ion attached. The reason for this is that

the bromonium ion blocks access to the carbon atoms along an entire side, due to bond

formation with the two carbon atoms. Such blocking is referred to as steric

hindrance. Because of steric hindrance, only a trans addition is possible.


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Alkenes: Hydrohalogenation

Unlike halogens, hydrogen halides are polarized molecules, which easily form ions.

Hydrogen halides also add to alkenes by electrophilic addition.

The addition of hydrogen halides to asymmetrically substituted alkenes leads to two

products.

The major product is predicted by the Markovnikov rule, which states that when a

hydrogen halide is added to an asymmetrically substituted alkene, the major product

results from the addition of the hydrogen atom to the double-bonded carbon that is

attached to more hydrogen atoms, while the halide ion adds to the other double-

bonded carbon. This arrangement creates a more stable carbocation intermediate.

Hydrohalogenation mechanisms. The first step in the addition of a hydrogen halide

to an alkene is the dissociation of the hydrogen halide.

The H+ion is attracted to the -bond electrons of the alkene, which forms a

complex.

The complex then breaks, creating a single bond between one carbon of the double-

bonded pair and the hydrogen. The carbon atom that loses a share of the bond thenbecomes a carbocation. In asymmetrically substituted alkenes, two different

carbocations are possible. The major product is generated from the more stable

carbocation, while the minor product forms from the less stable one.


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Thus, the major product is 2-bromopropane.

Hydrogen bromide can also be added to an alkene in an anti-Markovnikov fashion.

In anti-Markovnikov additions, the hydrogen atom of the hydrogen halide adds to

the carbon of the double bond that is bonded to fewer hydrogen atoms. For this to

result, the reaction must proceed by a noncarbocation intermediate; thus in the

presence of peroxide, the reaction proceeds via a free-radical mechanism, with the

major product being generated from the more stable free radical.

The mechanism for this reaction starts with the generation of a bromine free radical by

the reaction of hydrogen bromide with peroxide.

The bromine free radical adds to the alkene, forming a more stable carbon free radical.

The secondary free radical is more stable than the primary free radical because thesecondary molecule is better able to delocalize the stress placed on the carbon atom by

the free-radical electron. The major product then forms from the intermediates by

reacting with hydrogen bromide.


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In all additions of hydrogen halides across carbon-carbon double bonds, the majorproduct always comes from the more stable intermediate. In Markovnikov additions,

the major product results from the more stable carbocation, while in anti-Markovnikov

additions, such as the hydrogen bromide addition in the presence of peroxide, the

major product results from the more stable free radical.


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Alkenes: Catalytic Addition of Hydrogen

Hydrogenation is the addition of hydrogen to an alkene. Although this reaction is

exothermic, it is very slow. The addition of a metal catalyst, such as platinum,

palladium, nickel, or rhodium, greatly increases the reaction rate. Although this reactionseems simple, it is a highly complex addition. The reaction takes place in four steps.

In the first step, a hydrogen molecule reacts with the metal catalyst. This reaction

breaks the bond between the hydrogen atoms and creates weak hydrogen-metal

bonds. Next, the bond of an alkene molecule contacts the metal catalyst. The bond

is destroyed and two weak carbon-metal single bonds are created. Finally, the weakly

bound hydrogen atoms transfer one at a time from the catalyst surface to the carbon

atoms of the former alkene molecule, forming an alkane. Upon formation of the two

new carbon-hydrogen bonds, the alkane molecule can move away from the catalyst.

Because both of the added hydrogen atoms were bound to the surface of the catalyst,

they normally approach the alkene molecule from the same side, or face.This approach

of hydrogen atoms to the same face of an alkene molecule is called asyn addition.

When hydrogen atoms approach alkene molecules from opposite sides, the reaction is

called an antiaddition. Anti addition most likely occurs when double-bond

isomerization occurs more rapidly than the catalytic addition of the second hydrogen in

the hydrogenation.

Margarine manufacture

Some margarine is made by hydrogenating carbon-carbon double bonds inanimal or vegetable fats and oils. You can recognize the presence of this in

foods because the ingredients list will include words showing that it contains"hydrogenated vegetable oils" or "hydrogenated fats".

The impression is sometimes given that allmargarine is made byhydrogenation - that's simply not true.

Animal and vegetable fats and oils


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These are similar molecules, differing in their melting points. If the compoundis a solid at room temperature, you usually call it a fat. If it is a liquid, it is oftendescribed as an oil.

Their melting points are largely determined by the presence of carbon-carbon

double bonds in the molecule. The higher the number of carbon-carbondouble bonds, the lower the melting point.

If there aren't any carbon-carbon double bonds, the substance is said tobe saturated. A typical saturated fat might have the structure:

Molecules of this sort are usually solid at room temperature.

If there is only one carbon-carbon double bond in each of the hydrocarbonchains, it is called a mono-unsaturated fat (or mono-unsaturated oil,because it is likely to be a liquid at room temperature.)

A typical mono-unsaturated oil might be:


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If there are two or more carbon-carbon double bonds in each chain, then it is

said to be polyunsaturated.

For example:

For simplicity, in all these diagrams, all three hydrocarbon chains in each

molecule are the same. That doesn't have to be the case - you can have a

mixture of types of chain in the same molecule.

Making margarine

Vegetable oils often contain high proportions of polyunsaturated and mono-unsaturated fats (oils), and as a result are liquids at room temperature. Thatmakes them messy to spread on your bread or toast, and inconvenient forsome baking purposes.

You can "harden" (raise the melting point of) the oil by hydrogenating it in thepresence of a nickel catalyst. Conditions (like the precise temperature, or thelength of time the hydrogen is passed through the oil) are carefully controlledso that some, but not necessarily all, of the carbon-carbon double bonds arehydrogenated.

This produces a "partially hydrogenated oil" or "partially hydrogenated fat".

You need to hydrogenate enough of the bonds to give the final texture youwant. However, there are possible health benefits in eating mono-unsaturatedor polyunsaturated fats or oils rather than saturated ones - so you wouldn'twant to remove all the carbon-carbon double bonds.

The flow diagram below shows the complete hydrogenation of a typical mono-unsaturated oil.


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The downside of hydrogenation as a means of hardening fats and oils

There are some probable health risks from eating hydrogenated fats or oils.Consumers are becoming more aware of this, and manufacturers areincreasingly finding alternative ways of converting oils into spreadable solids.

One of the problems arises from the hydrogenation process.

The double bonds in unsaturated fats and oils tend to have the groups aroundthem arranged in the "cis" form.

The relatively high temperatures used in the hydrogenation process tend toflip some of the carbon-carbon double bonds into the "trans" form. If theseparticular bonds aren't hydrogenated during the process, they will still bepresent in the final margarine in molecules of trans fats.

The consumption of trans fats has been shown to increase cholesterol levels(particularly of the more harmful LDL form) - leading to an increased risk ofheart disease.


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Any process which tends to increase the amount of trans fat in the diet is bestavoided. Read food labels, and avoid any food which contains (or is cookedin) hydrogenated oil or hydrogenated fat.

organic chemistry: alkene notes

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