Unsaturated Chain Hydrocarbons

Overview

Unsaturated chain hydrocarbons are hydrocarbons whose carbon skeleton contains at least one carbon–carbon multiple bond (double or triple bond). In contrast to saturated chain hydrocarbons, they do not have the maximum possible number of hydrogen atoms per carbon.

This chapter focuses on:

• Structural types (alkenes and alkynes)
• Consequences of multiple bonds for geometry and physical properties
• Typical reactions characteristic of unsaturated chains
• Some important examples and applications

General ideas of organic structure, bonding, and basic reactivity are assumed from earlier chapters.

Classification and General Formulas

Alkenes (Olefins)

Alkenes contain at least one carbon–carbon double bond $C=C$.

• Simplest homologous series with one double bond and no rings:

$$\text{General formula: } C_nH_{2n} \quad (n \ge 2)$$

• Examples:
• Ethene (ethylene): $C_2H_4$
• Propene (propylene): $C_3H_6$
• 1‑butene: $C_4H_8$

If more than one double bond is present:

• Dienes: two double bonds
• Trienes: three double bonds
• Polyenes: many double bonds (e.g. in some natural dyes and vitamins)

Alkynes

Alkynes contain at least one carbon–carbon triple bond $C\equiv C$.

• Simplest homologous series with one triple bond and no rings:

$$\text{General formula: } C_nH_{2n-2} \quad (n \ge 2)$$

• Examples:
• Ethyne (acetylene): $C_2H_2$
• Propyne: $C_3H_4$
• 1‑butyne: $C_4H_6$

As with alkenes, multiple triple bonds (diynes, triynes) and mixed systems (ene‑ynes) are possible.

Structure and Bonding

Hybridization and Bond Types

At the atoms forming the multiple bond:

• In alkenes:
• The two carbon atoms of the $C=C$ bond are $sp^2$‑hybridized.
• One $\sigma$ bond is formed by overlap of $sp^2$ orbitals.
• One $\pi$ bond is formed by side‑by‑side overlap of unhybridized $p$ orbitals.
• In alkynes:
• The two carbon atoms of the $C\equiv C$ bond are $sp$‑hybridized.
• One $\sigma$ bond is formed by overlap of $sp$ orbitals.
• Two $\pi$ bonds are formed by two sets of side‑by‑side $p$‑orbital overlaps.

Geometry

• Alkenes:
• The $sp^2$‑hybridized carbon is approximately trigonal planar.
• Ideal bond angles are about $120^\circ$ at each carbon of the double bond.
• The four atoms directly involved in the double bond (e.g. $C_1, H, C_2, H$ in ethene) lie approximately in one plane.
• Alkynes:
• The $sp$‑hybridized carbon is linear.
• Ideal bond angle is $180^\circ$ at each carbon of the triple bond.
• The three atoms directly involved (e.g. $H–C\equiv C–H$ in ethyne) lie on a straight line.

Restricted Rotation and Stereochemistry at Double Bonds

Rotation around a $C=C$ double bond would require breaking the $\pi$ bond, so under normal conditions rotation is restricted. This leads to stereoisomerism:

• If each carbon of the $C=C$ bond carries two different substituents, two configurations are possible:
• Substituents on the same side (cis or Z, depending on the notation system)
• Substituents on opposite sides (trans or E)

Example: 2‑butene, $CH_3–CH=CH–CH_3$:

• cis‑2‑butene: the two $CH_3$ groups on the same side
• trans‑2‑butene: the two $CH_3$ groups on opposite sides

Alkynes, being linear at the triple bond, do not show analogous cis/trans isomerism at the $C\equiv C$ itself.

Nomenclature Aspects Specific to Unsaturated Chains

(General rules of IUPAC naming are handled elsewhere; here only features special to multiple bonds are highlighted.)

Numbering and Suffixes

• Alkenes: suffix ‑ene
• Longest chain containing the double bond is chosen as the parent chain.
• Chain is numbered so that the first carbon of the double bond has the lowest possible number.
• Position of the double bond is indicated by the number of the first double‑bond carbon.

Examples:

• Propene: $CH_3–CH=CH_2$ (only one possible structure)
• 1‑butene: $CH_2=CH–CH_2–CH_3$
• 2‑butene: $CH_3–CH=CH–CH_3$

• Alkynes: suffix ‑yne
• Analogous rules: choose the chain with the triple bond and number from the end giving the triple bond the lowest number.

Examples:

• 1‑butyne: $CH\equiv C–CH_2–CH_3$
• 2‑butyne: $CH_3–C\equiv C–CH_3$

Multiple Multiple Bonds

In names:

• ‑diene, ‑triene, ‑tetrayne, etc. for multiple double or triple bonds.
• Positions of all multiple bonds are indicated.

Example: 1,3‑butadiene: $CH_2=CH–CH=CH_2$

Stereodescriptors for Alkenes

For alkenes capable of stereoisomerism:

• cis/trans notation is often used for simple cases (like 2‑butene).
• The more general E/Z system is based on priority rules for substituents.
  Details of these systems are treated with stereochemistry elsewhere; here it suffices that unsaturated chains can have configurational isomers around $C=C$.

Physical Properties and Trends

Polarity and Intermolecular Forces

• Simple alkenes and alkynes (hydrocarbons only) are nonpolar or only very weakly polar.
• Intermolecular forces are dominated by London dispersion forces.
• Boiling and melting points increase with chain length, similar to alkanes:
• More electrons and larger surface ⇒ stronger dispersion forces.
• For a given carbon number, boiling points of alkenes and alkynes are usually close to those of the corresponding alkanes, with small differences due to shape and polarizability.

Density and Solubility

• Like other hydrocarbons, alkenes and alkynes:
• Are less dense than water.
• Are practically insoluble in water.
• Are soluble in many organic solvents (e.g. hexane, benzene, ether).

Influence of the Double Bond on Shape and Packing

• Cis vs trans isomers show different physical properties:
• trans isomers are often more symmetrical, pack better in the crystal lattice, and can have higher melting points.
• cis isomers can be more polar (due to geometry) and have slightly different boiling points and solubilities.

Reactivity: General Features of Unsaturated Chains

Electron Density and Reactivity of Multiple Bonds

• $C=C$ and $C\equiv C$ bonds have higher electron density than single bonds.
• The $\pi$ electrons are more exposed and more easily attacked by electrophiles (electron‑poor species).
• As a result, alkenes and alkynes are typically more reactive than analogous alkanes.

Addition vs Substitution

A key contrast to saturated hydrocarbons:

• Alkanes: mostly undergo substitution reactions (one atom or group is replaced by another, often requiring strong conditions).
• Alkenes and alkynes: characteristically undergo addition reactions at the multiple bond:
• At a double bond: addition converts $C=C$ into $C–C$ (an alkane‑like fragment).
• At a triple bond: two successive additions can convert $C\equiv C$ to $C–C$.

General scheme for a simple addition to an alkene:

$$\ce{RCH=CHR' + X–Y -> RCHX–CHYR'}$$

where $X–Y$ could be $H_2$, $HX$, $X_2$, $H_2O$, etc.

Typical Reactions of Alkenes

Detailed mechanisms and kinetics are covered in reaction‑type chapters; here we outline the main classes of reactions for unsaturated chains.

1. Hydrogenation (Addition of Hydrogen)

Hydrogenation is the addition of $H_2$ to the $C=C$ bond, usually with a metal catalyst (e.g. Ni, Pd, Pt):

$$\ce{RCH=CHR' + H2 ->[Ni] RCH2–CH2R'}$$

• Converts alkenes to alkanes.
• Industrially important in hardening of vegetable oils (partial hydrogenation of polyunsaturated fatty acids).

2. Halogenation (Addition of Halogens)

Halogens $X_2$ (typically $Cl_2$ or $Br_2$) add across the double bond to give dihalogenated products:

$$\ce{RCH=CHR' + X2 -> RCHX–CHXR'}$$

• Occurs readily at room temperature.
• Used as a qualitative test for unsaturation:
• Decolorization of bromine solution indicates presence of double or triple bonds.

3. Hydrohalogenation (Addition of Hydrogen Halides)

Addition of $HX$ (e.g. $HCl$, $HBr$) across the double bond yields alkyl halides:

$$\ce{RCH=CHR' + HX -> RCHX–CH2R'}$$

For unsymmetrical alkenes, the distribution of $H$ and $X$ follows Markovnikov’s rule, which can be summarized qualitatively as:

• The hydrogen atom adds to the carbon with more hydrogens already attached.
• The halogen adds to the more substituted carbon of the double bond.

Regioselectivity and exceptions (e.g. anti‑Markovnikov addition) are discussed in reaction‑mechanism chapters.

4. Hydration (Addition of Water)

In the presence of an acid catalyst (e.g. $H_2SO_4$), water adds across the double bond:

$$\ce{RCH=CHR' + H2O ->[\text{H+}] RCH(OH)–CH2R'}$$

• Forms alcohols.
• In unsymmetrical alkenes, the OH group usually attaches to the more substituted carbon (Markovnikov orientation).

5. Oxidation Reactions

Selected oxidation types:

• Mild oxidation (e.g. cold, dilute $KMnO_4$):
• Can convert alkenes to diols (vicinal glycols):

$$\ce{RCH=CHR' + [O] -> RCH(OH)–CH(OH)R'}$$

• Strong oxidative cleavage (e.g. hot, concentrated $KMnO_4$ or ozonolysis followed by oxidative work‑up):
• Breaks the double bond to form carbonyl compounds (aldehydes, ketones, or carboxylic acids, depending on the substrate and conditions).

The exact products and mechanisms are treated in detail with carbonyl chemistry and oxidation reactions.

6. Polymerization of Alkenes

Many simple alkenes undergo addition polymerization:

$$\ce{n CH2=CH2 ->[-\,\ce{CH2–CH2}-]_n}$$

• Ethene → polyethylene (PE).
• Propene → polypropylene (PP).
• Vinyl chloride ($CH_2=CHCl$) → poly(vinyl chloride) (PVC).

Polymerization mechanisms (radical, ionic, coordination) and materials properties are discussed extensively in polymer chemistry chapters; here the key point is that the $C=C$ bond allows chain‑growth reactions leading to high‑molecular‑weight products.

Typical Reactions of Alkynes

Alkynes show many parallels to alkenes but also have features arising from the triple bond and terminal $C–H$ acidity.

1. Stepwise Addition to the Triple Bond

The triple bond can add reagents in two steps:

Hydrogenation

• Partial hydrogenation:

$$\ce{R–C#C–R' + H2 ->[Pd/BaSO4, \; poison] RCH=CHR'}$$

• Stops at the alkene stage under controlled conditions (e.g. Lindlar’s catalyst) and usually gives cis‑alkenes.

• Complete hydrogenation:

$$\ce{R–C#C–R' + 2 H2 ->[Ni] RCH2–CH2R'}$$

• Converts alkynes all the way to alkanes.

Addition of Halogens and Hydrogen Halides

• Similar to alkenes, but additions occur twice if sufficient reagent is present:
• With $X_2$:

$$\ce{R–C#C–R' + 2 X2 -> RX2–C–C–X2R'}$$

• With $HX$:

$$\ce{R–C#C–R' + 2 HX -> RCHX2–CH2XR'}$$

Regioselectivity (Markovnikov vs anti‑Markovnikov) is also relevant here and is addressed with reaction mechanisms.

2. Hydration of Alkynes

Acid‑catalyzed hydration of alkynes (often with $Hg^{2+}$ catalysts):

• First forms an enol (a compound with $C=C$ and $OH$ on the same carbon).
• The enol then rearranges (tautomerizes) to a carbonyl compound:
• Terminal alkynes → aldehydes
• Internal alkynes → ketones

Example (simplified):

$$\ce{HC#CH + H2O ->[\text{H+}, Hg^{2+}] CH3–CHO}$$

(enol intermediate not shown; final product is ethanal/acetaldehyde)

3. Acidity of Terminal Alkynes and Nucleophilic Reactions

Terminal alkynes (with $C\equiv C–H$) have a relatively acidic hydrogen (more acidic than $C–H$ in alkanes and alkenes):

• Strong bases (e.g. $NaNH_2$) can deprotonate them to form acetylide anions:

$$\ce{RC#CH + Base^- -> RC#C^- + HB}$$

• These acetylide anions are good nucleophiles and can:
• Attack electrophilic carbon centers (e.g. in haloalkanes),
• Form new $C–C$ bonds, extending the carbon chain.

This makes terminal alkynes valuable building blocks in organic synthesis.

Conjugated and Isolated Double Bonds in Chains

Unsaturated chain hydrocarbons can have:

• Isolated double bonds:
• Double bonds separated by at least one $sp^3$ carbon.
• Example: $CH_2=CH–CH_2–CH=CH_2$ (1,4‑pentadiene).
• Conjugated double bonds:
• Alternating pattern of single and double bonds ($C=C–C=C$).
• Example: 1,3‑butadiene, $CH_2=CH–CH=CH_2$.

Conjugation has several consequences:

• Delocalization of $\pi$ electrons over several atoms.
• Usually lower reactivity of each individual double bond compared with an isolated one, but new reaction patterns become possible (e.g. specific addition modes, cycloaddition reactions).
• Often affects color (in long conjugated chains) and stability.

Details of conjugation, resonance, and their spectroscopy and reactivity implications are discussed in the chapters on electronic effects and aromatic systems.

Industrial and Everyday Importance

Unsaturated chain hydrocarbons play central roles in technology and daily life:

• Ethene:
• Large‑scale monomer for polyethylene, ethylene oxide, ethylene glycol, etc.
• Used in agriculture (plant hormone influencing fruit ripening).
• Propene:
• Monomer for polypropylene.
• Starting material for industrial chemicals like isopropanol and acrylonitrile.
• 1,3‑Butadiene:
• Key monomer for synthetic rubbers (e.g. styrene‑butadiene rubber).
• Ethyne (acetylene):
• Used historically as a fuel for oxyacetylene welding.
• Precursor for various chemicals via addition and polymerization reactions.
• Unsaturated fatty acids (long‑chain alkenes in natural fats and oils):
• Important in nutrition and biology.
• Degree of unsaturation influences melting points (liquid oils vs solid fats) and susceptibility to oxidation (rancidity).

These examples illustrate how the presence and arrangement of multiple bonds in carbon chains determine not only their chemistry in the laboratory but also their technical and biological roles.

Summary of Key Points

• Unsaturated chain hydrocarbons include:
• Alkenes (with $C=C$)
• Alkynes (with $C\equiv C$)
• Multiple bonds:
• Involve $\sigma$ and $\pi$ components.
• Lead to characteristic geometries ($sp^2$ planar for $C=C$, $sp$ linear for $C\equiv C$).
• Restrict rotation in alkenes, enabling cis/trans (E/Z) isomerism.
• Physical properties:
• Generally nonpolar, low density, poor water solubility.
• Boiling and melting points depend mainly on chain length and, for alkenes, stereochemistry.
• Reactivity:
• Dominated by electrophilic additions to the multiple bond (hydrogenation, halogenation, hydrohalogenation, hydration, oxidation).
• Alkynes can undergo stepwise additions and show notable acidity at terminal $C\equiv C–H$.
• Many alkenes can polymerize to form important plastics and elastomers.
• Structural variations (isolated vs conjugated multiple bonds) significantly influence stability, reactivity, and, in extended systems, color and electronic properties.