What is the primary difference in bonding between alkanes and alkenes?

The fundamental difference lies in the carbon-carbon bonds. Alkanes contain only carbon-carbon single bonds, which allow free rotation around the bond axis. Alkenes, on the other hand, possess at least one carbon-carbon double bond. This double bond consists of a strong sigma ($\sigma$) bond and a weaker pi ($\pi$) bond. The presence of the pi bond restricts rotation around the $C=C$ axis and makes alkenes electron-rich, leading to their characteristic electrophilic addition reactions, which are not typical for alkanes.

Why do alkenes undergo addition reactions, while alkanes undergo substitution reactions?

Alkenes are unsaturated due to the presence of a pi ($\pi$) bond, whose electrons are relatively exposed and loosely held. This makes the pi bond a site of high electron density, readily attacked by electron-deficient species (electrophiles). The pi bond breaks, and new atoms or groups add across the double bond, converting the alkene into a saturated product. Alkanes, being saturated, have only strong C-C and C-H sigma bonds. Breaking these bonds requires significant energy, and they are not easily attacked by electrophiles. Instead, alkanes typically undergo substitution reactions, where a hydrogen atom is replaced by another atom or group, usually via a free radical mechanism under harsh conditions (e.g., halogenation in UV light).

Explain Markovnikov's rule with an example.

Markovnikov's rule is a regioselectivity rule in organic chemistry, particularly for the addition of unsymmetrical reagents (like HX or $H_2O$) to unsymmetrical alkenes. It states that the hydrogen atom of the reagent adds to the carbon atom of the double bond that already has a greater number of hydrogen atoms, while the electronegative part adds to the carbon atom with fewer hydrogen atoms. For example, in the addition of HBr to propene ($CH_3-CH=CH_2$), the hydrogen of HBr adds to the $CH_2$ carbon, and bromine adds to the $CH$ carbon, yielding 2-bromopropane ($CH_3-CHBr-CH_3$) as the major product. This is because the reaction proceeds via the formation of the more stable secondary carbocation intermediate.

What is the significance of the peroxide effect in alkene reactions?

The peroxide effect, also known as the Kharasch effect, is a specific phenomenon observed during the addition of HBr (and only HBr, not HCl or HI) to unsymmetrical alkenes in the presence of organic peroxides. It leads to an 'anti-Markovnikov' addition, meaning the hydrogen adds to the carbon with fewer hydrogens, and the bromine adds to the carbon with more hydrogens. This occurs via a free radical mechanism, where the peroxide initiates the formation of bromine radicals, which then add to the alkene to form the more stable carbon radical, ultimately leading to the anti-Markovnikov product. This effect is crucial for synthesizing 1-bromoalkanes from terminal alkenes.

How can you distinguish between an alkene and an alkane using simple chemical tests?

Two common tests can distinguish alkenes from alkanes, both relying on the alkene's unsaturation. The **Bromine Water Test** involves adding bromine water (reddish-brown) to the sample. Alkenes will rapidly decolorize the bromine water as bromine adds across the double bond, forming a colorless vicinal dibromide. Alkanes, being saturated, do not react with bromine water in the absence of UV light, so the color persists. The **Baeyer's Test** uses cold, dilute, alkaline potassium permanganate solution (purple). Alkenes will decolorize the purple solution and form a brown precipitate of manganese dioxide ($MnO_2$) as they are oxidized to diols. Alkanes do not react under these mild conditions, and the purple color remains.

What is ozonolysis and why is it important?

Ozonolysis is a powerful reaction used to cleave carbon-carbon double bonds in alkenes (and triple bonds in alkynes) using ozone ($O_3$), followed by a reductive or oxidative workup. The alkene first reacts with ozone to form an unstable ozonide intermediate, which is then cleaved. Reductive workup (e.g., with $Zn/H_2O$ or $Me_2S$) yields aldehydes and ketones. Oxidative workup (e.g., with $H_2O_2$) converts any aldehydes formed into carboxylic acids, while ketones remain unchanged. Ozonolysis is incredibly important for determining the position of the double bond in an unknown alkene, as the products formed directly reveal the structure of the original alkene.

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Alkenes

Chemistry

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Version 1Updated 22 Mar 2026

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Alkenes are a class of unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond ( $C=C$ ) in their molecular structure. Their general molecular formula for acyclic alkenes with one double bond is $C_nH_{2n}$ , where 'n' represents the number of carbon atoms and must be greater than or equal to 2. The double bond consists of one sigma ( $\sigma$ ) bond and one pi …

Quick Summary

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond ( $C=C$ ). Their general formula is $C_nH_{2n}$ . Each carbon in the double bond is $sp^2$ hybridized, resulting in a trigonal planar geometry and restricted rotation, which gives rise to cis-trans isomerism.

The double bond consists of a strong sigma ( $\sigma$ ) bond and a weaker, more exposed pi ( $\pi$ ) bond, making alkenes electron-rich and highly reactive towards electrophiles. \n\nNomenclature involves finding the longest chain containing the double bond, numbering to give the double bond the lowest locant, and changing the '-ane' suffix to '-ene'.

\n\nAlkenes are prepared by elimination reactions like dehydration of alcohols and dehydrohalogenation of alkyl halides (often following Saytzeff's rule), or by partial hydrogenation of alkynes. \n\nTheir characteristic reactions are electrophilic additions, including hydrogenation, halogenation (anti-addition), hydrohalogenation (Markovnikov's rule, or anti-Markovnikov with HBr/peroxides), and hydration.

They also undergo oxidation (Baeyer's test, ozonolysis) and polymerization. These reactions are fundamental to their industrial applications, such as the production of plastics like polyethylene and polypropylene, and their role in natural processes like fruit ripening.

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Key Concepts

Geometrical (cis-trans) Isomerism

Geometrical isomerism, also known as cis-trans isomerism, arises in alkenes due to the restricted rotation…

Markovnikov's Rule and Carbocation Stability

Markovnikov's rule governs the regioselectivity of electrophilic addition to unsymmetrical alkenes. It states…

Saytzeff's Rule in Elimination Reactions

Saytzeff's rule (also spelled Zaitsev's rule) is an empirical rule used to predict the major product in…

General Formula: Acyclic alkenes

C_nH_{2n}

.\n- Double Bond: One

\sigma

and one

\pi

bond. Restricted rotation.\n- Hybridization:

sp^2

for double-bonded carbons, trigonal planar geometry (

120^\circ

).\n- Nomenclature: '-ene' suffix, lowest locant for double bond.\n- Isomerism: Structural (chain, position, functional with cycloalkanes), Geometrical (cis-trans, requires two different groups on each C of

C=C

).\n- Preparation:\n - Dehydration of alcohols (

H_2SO_4/\Delta

): Saytzeff's rule.\n - Dehydrohalogenation of alkyl halides (Alc. KOH/

\Delta

): Saytzeff's rule.\n - Dehalogenation of vicinal dihalides (

Zn/Alcohol

).\n - Partial hydrogenation of alkynes:\n - Lindlar's catalyst (

H_2/Pd/CaCO_3

, quinoline/sulfur): *cis*-alkene (syn).\n -

Na/liq.\,NH_3

: *trans*-alkene (anti).\n- Reactions (Electrophilic Addition):\n - Hydrogenation (

H_2/Ni, Pt, Pd

): Syn addition, forms alkane.\n - Halogenation (

X_2/CCl_4

): Anti addition, forms vicinal dihalide. Decolorizes

Br_2

water.\n - Hydrohalogenation (

HX

): Markovnikov's rule (H to C with more H, X to C with fewer H). Carbocation intermediate.\n - Anti-Markovnikov (

HBr/Peroxide

): Free radical mechanism, H to C with fewer H, Br to C with more H.\n - Hydration (

H_2O/H^+

): Markovnikov's rule, forms alcohol.\n- Oxidation:\n - Baeyer's test (Cold, dil., alk.

KMnO_4

): Syn addition, forms vicinal diol (glycol). Decolorizes

KMnO_4

, brown

MnO_2

ppt.\n - Ozonolysis (

O_3

then

Zn/H_2O

H_2O_2

): Cleaves

C=C

bond, forms aldehydes/ketones (reductive) or carboxylic acids/ketones (oxidative).\n- Polymerization: Addition polymerization (e.g., polyethylene).

Markovnikov Has More Hydrogens, Anti-Markovnikov Has Less Hydrogens (for HBr/peroxide). \n\nThis mnemonic helps remember the regioselectivity of HBr addition to unsymmetrical alkenes.

'Markovnikov Has More Hydrogens' means the hydrogen of HBr adds to the carbon of the double bond that already has more hydrogen atoms. 'Anti-Markovnikov Has Less Hydrogens' means, in the presence of peroxide, the hydrogen of HBr adds to the carbon of the double bond that has fewer hydrogen atoms.

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