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Alkenes

Knowledge about the structure and function of alkenes. For example, alkenes with many substituents have high π electron cloud density and high electrophilic addition reaction activity; if there are electron-withdrawing groups, electron-withdrawing induction effects will occur, reducing the π electron cloud density and electrophilic addition reaction. The closer the electron-withdrawing group is to the double bond, the greater the influence.

Edited at 2023-10-20 19:59:24

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Alkenes

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Organic Chemistry Alkenes Mind Map
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Alkynes reaction
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Alkenes mind map
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Alkenes

Comparison of the characteristics of σ bond and π bond

σ bond

Can exist alone in covalent bonds

Overlap along the symmetry axis to form a bond, with a large degree of overlap

The electron cloud is cylindrical

The electron cloud density is the largest on the symmetry axis and is concentrated between the two nuclei.

π bond

Cannot exist alone and coexist with σ bond

Overlap parallel to each other along the axis of symmetry, with a small degree of overlap

The electron cloud is lumpy and has a symmetry plane

The electron cloud density on the symmetry plane is almost zero, and the electron cloud is relatively dispersed.

cis-trans isomers

Due to the obstruction of rotation around the π bond, the atoms or groups in the molecule are arranged in different positions in space, and the isomers produced are called cis-trans isomers or geometric isomers.

cis isomer

Identical groups are on the same side of the double bond carbon

trans isomer

The same groups are on opposite sides of the double bond carbon

The same group is located on the same carbon atom

No cis-trans isomerism

Nomenclature of cis-trans isomers

Cis/trans configuration notation

cis

Trans(trans)

Z/E configuration notation

Z-

Priority groups are on the same side of the double bond carbon

E-

The priority group is located on the opposite side of the double bond carbon

physical properties

Among the cis and trans isomers of alkenes, the cis form has a higher boiling point than the trans form.

The trans form has higher symmetry, so the melting point of the trans form is generally higher than that of the cis form.

chemical properties

Catalytic hydrogenation reaction

The more substituents on the double bond carbon, the greater the steric hindrance, the more stable the alkene, and the less likely it is to undergo catalytic hydrogenation.

Under the catalysis of platinum (pt), palladium (pd), nickel (Ni), etc., it is added with hydrogen to generate alkanes

Belongs to reduction reaction

for cis addition

Alkenes with small steric hindrance are easy to add

electrophilic addition reaction

An addition reaction in which an electrophile attacks the π electron cloud of a double bond

Addition to hydrogen halide

Comply with "Markov Rules"

Hydrogen multi-hydrogenation

The order of activity of hydrogen halides

HI>HBr>HCl

Addition with sulfuric acid

Alkenes react with cold concentrated sulfuric acid to form hydrogen sulfate esters, which can be further hydrolyzed to form alcohols.

Comply with "Markov Rules"

Add to water

Under the catalysis of inorganic acids (such as dilute sulfuric acid and dilute phosphoric acid), alkenes react with water to form alcohols

Comply with "Markov Rules"

Addition with halogen

Addition of alkenes to halogens produces o-dihaloalkane

for trans addition

The order of reactivity of halogens

F2>Cl2>Br2>I2

Addition with hypohalous acid

Alkenes react with hypohalous acid (aqueous solution of halogen) to form o-halohydrins

Halogens are added to carbon atoms containing more hydrogen

for trans addition

hydroboration reaction

Borane reacts with alkenes in ether solvents to form alkylborane

Hydroboration-oxidation reaction

Trialkyl boron is oxidized with H2O2 under alkaline conditions, and the C-B bond is broken to generate 3 molecules of alcohol.

Comply with the "Anti-Markov Rules"

for cis addition

Borane can easily polymerize to diborane, which can be decomposed into borane in ether solvents.

free radical addition reaction

Free radical substitution reaction and addition reaction are essentially a pair of competitive reactions. Low halogen concentration is conducive to the substitution reaction.

peroxide effect

In the presence of peroxide, an addition reaction occurs between alkenes and hydrogen bromide to produce "anti-Markov's rule" products.

The peroxide effect is effective for HBr but not for HI and HCl

Comply with the "Anti-Markov Rules"

oxidation reaction

Potassium permanganate or osmium tetroxide oxidation

Alkenes undergo an oxidation reaction with hot, concentrated acidic potassium permanganate, and the carbon-carbon double bonds break.

There is no hydrogen on the double bonded carbon

Generate ketones

There is a hydrogen on the double bonded carbon

generate carboxylic acid

There are two hydrogens on a double bonded carbon

Generate formic acid, which is unstable and continues to be oxidized to generate carbon dioxide.

Alkenes react with cold, dilute potassium permanganate aqueous solution, the olefins are oxidized to cis-diol, and potassium permanganate is reduced to manganese dioxide

ozone oxidation

Alkenes react with ozone to form ozonides, which are treated with water or acid (Zn, H2O) in the presence of a reducing agent to form aldehydes or ketones.

peroxyacid oxidation

Alkenes react with peroxyacids to form epoxides

Halogenation reaction of α-hydrogen

Under light or high temperature conditions, α-hydrogen is replaced by halogen to generate halogenated alkenes

The carbon atom adjacent to the carbon-carbon double bond is an α-carbon atom

Polymerization

Under the action of a catalyst or initiator, olefins are polymerized to form macromolecular compounds

Stability of carbocation

Tertiary carbocation > Secondary carbocation > Primary carbocation > Methyl carbocation

Free radical stability

Allyl radical > Tertiary carbon radical > Secondary carbon radical > Primary carbon radical > Ethylene radical

induction effect

Due to the different electronegativities of the bonding atoms in the molecule, the covalent bond electron cloud is no longer evenly distributed between the two atoms, but is biased from the less electronegative atom to the more electronegative atom along the carbon chain. Conveys the effect of electrostatic induction

Electron-withdrawing induction effect (-I)

electron withdrawing group

Reduce π electron cloud density

Electron donation induced effect (I)

electron donating group

Alkyl groups are electron donating groups

Increase π electron cloud density

cis addition reaction

Catalytic hydrogenation reaction

Hydroboration-oxidation reaction

Alkenes react with cold, dilute aqueous potassium permanganate solution

trans addition reaction

Addition with halogen

Addition with hypohalous acid

Reactions that comply with the anti-Markov rule

Hydroboration-oxidation reaction

peroxide effect

Alkenes with many substituents have high π electron cloud density and high electrophilic addition reaction activity; if there are electron-withdrawing groups, electron-withdrawing induction effects will occur, reducing the π electron cloud density and electrophilic addition reaction activity. The closer to the double bond, the greater the influence