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A topic from the subject of Organic Chemistry in Chemistry.

  • 1 year ago
  • 9 min read

Reactions of Alkanes and Alkenes

Introduction

Alkanes and alkenes are two important classes of hydrocarbons that differ significantly in their reactivity. Alkanes are saturated hydrocarbons, meaning that all of their carbon atoms are bonded to four other atoms by single bonds. Alkenes, on the other hand, are unsaturated hydrocarbons, containing at least one carbon-carbon double bond. This double bond makes alkenes much more reactive than alkanes.

Basic Concepts

  • Alkanes are hydrocarbons with the general formula CnH2n+2. They are saturated, meaning that all carbon atoms are bonded to four other atoms via single bonds.
  • Alkenes are hydrocarbons with the general formula CnH2n. They are unsaturated, meaning they have at least one carbon-carbon double bond.

Reactions of Alkanes

Alkanes are relatively unreactive due to the strong C-C and C-H single bonds. Their primary reactions are free radical substitution reactions, which require high temperatures or UV light. A common example is halogenation (reaction with halogens like chlorine or bromine).

Example: CH4 + Cl2 → CH3Cl + HCl (in the presence of UV light)

Reactions of Alkenes

Alkenes are much more reactive than alkanes due to the presence of the electron-rich carbon-carbon double bond. They undergo addition reactions, where atoms or groups of atoms add across the double bond. Common addition reactions include:

  • Halogenation: Addition of halogens (e.g., Br2, Cl2) across the double bond.
  • Hydrogenation: Addition of hydrogen (H2) across the double bond in the presence of a catalyst (e.g., Pt, Pd, Ni).
  • Hydration: Addition of water (H2O) across the double bond, forming an alcohol. This often requires an acid catalyst.
  • Hydrohalogenation: Addition of hydrogen halides (e.g., HCl, HBr) across the double bond.
  • Polymerization: Alkenes can undergo addition polymerization to form long chains called polymers (e.g., polyethylene from ethene).

Equipment and Techniques

The following equipment and techniques are commonly used to study the reactions of alkanes and alkenes:

  • Gas chromatography (GC)
  • Mass spectrometry (MS)
  • Nuclear magnetic resonance (NMR) spectroscopy
  • Infrared (IR) spectroscopy
  • Ultraviolet-visible (UV-Vis) spectroscopy
  • High-performance liquid chromatography (HPLC)

Data Analysis

Data from experiments can be used to determine:

  • The identity of the reaction products
  • The reaction mechanism
  • The reaction rate
  • The reaction equilibrium constant

Applications

The reactions of alkanes and alkenes are used in various industrial processes, including:

  • The production of plastics
  • Petroleum refining
  • Pharmaceutical manufacturing
  • The synthesis of new materials

Conclusion

The reactions of alkanes and alkenes are fundamental in organic chemistry. Understanding their mechanisms allows for the design of efficient processes for producing various important products.

Reactions of Alkanes and Alkenes

Alkanes

Alkanes are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms. They are relatively unreactive due to the stability of the carbon-carbon and carbon-hydrogen bonds. Their reactions typically involve bond breaking and are often free radical reactions.

Combustion:

Alkanes react with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O). This is a highly exothermic reaction, releasing significant heat. A general equation is: CnH2n+2 + O2 → CO2 + H2O (unbalanced)

Halogenation:

Alkanes can react with halogens (e.g., chlorine (Cl2), bromine (Br2)) in the presence of ultraviolet (UV) light or heat. This is a free radical substitution reaction, leading to the formation of alkyl halides. For example, methane reacting with chlorine:

CH4 + Cl2 → CH3Cl + HCl

Free Radical Reactions:

Alkanes can undergo other free radical reactions, such as cracking (breaking down larger alkanes into smaller ones) and polymerization (though less common for alkanes than alkenes).

Alkenes

Alkenes are unsaturated hydrocarbons, meaning they contain at least one carbon-carbon double bond. The presence of the double bond makes them significantly more reactive than alkanes. Reactions often involve the addition of atoms or groups to the double bond.

Addition Reactions:

Alkenes readily undergo addition reactions, where atoms or groups are added across the double bond, breaking the π bond and resulting in a saturated product.

  • Hydrogenation: Addition of hydrogen (H2) in the presence of a catalyst (like nickel or platinum) to form an alkane. For example, ethene hydrogenating to ethane:

    • CH2=CH2 + H2 → CH3CH3

  • Halogenation: Addition of halogens (Cl2, Br2, I2) to form vicinal dihalides (halogens on adjacent carbons). For example, ethene reacting with bromine:

    • CH2=CH2 + Br2 → CH2BrCH2Br

  • Hydrohalogenation: Addition of hydrogen halides (HCl, HBr, HI) to form alkyl halides. Markovnikov's rule applies, predicting the hydrogen atom adds to the carbon with more hydrogens already attached. For example, propene reacting with HBr:

    • CH3CH=CH2 + HBr → CH3CHBrCH3

  • Hydration: Addition of water (H2O) in the presence of an acid catalyst (like H2SO4) to form an alcohol. Markovnikov's rule also applies. For example, ethene hydrating to ethanol:

    • CH2=CH2 + H2O → CH3CH2OH

Polymerization:

Alkenes can undergo addition polymerization, where many alkene monomers add together to form long chains called polymers. Examples include polyethylene (from ethene) and polypropylene (from propene).

Oxidation:

Alkenes can be oxidized by strong oxidizing agents such as potassium permanganate (KMnO4) or ozone (O3). This can lead to the formation of epoxides or diols (glycols).

Experiment: Reactions of Alkanes and Alkenes

Objective:

To investigate the different reactions of alkanes and alkenes.

Materials:

  • Methane gas
  • Ethene gas
  • Bromine in carbon tetrachloride
  • Potassium permanganate solution
  • Hydrochloric acid
  • Sodium hydroxide
  • Phenolphthalein indicator
  • Test tubes
  • Bunsen burner (for methane/ethene delivery if not pre-piped)

Procedure:

1. Reaction of methane with bromine:

  1. Place a few drops of bromine in carbon tetrachloride in a test tube.
  2. Bubble methane gas through the solution. (Note: This reaction requires UV light or significant heating to proceed at a reasonable rate.)
  3. Observe the reaction (or lack thereof).

2. Reaction of ethene with bromine:

  1. Repeat step 1, but use ethene gas instead of methane.
  2. Observe the reaction.

3. Reaction of ethene with potassium permanganate:

  1. Place a few drops of potassium permanganate solution in a test tube.
  2. Bubble ethene gas through the solution.
  3. Observe the reaction (color change).

4. Reaction of alkanes with hydrochloric acid:

  1. Place a few drops of an alkane (e.g., hexane) in a test tube.
  2. Add a few drops of hydrochloric acid.
  3. Observe the reaction (or lack thereof).

5. Reaction of alkenes with sodium hydroxide:

  1. Place a few drops of an alkene (e.g., hexene) in a test tube.
  2. Add a few drops of sodium hydroxide solution.
  3. Add a drop of phenolphthalein indicator.
  4. Observe the reaction (or lack thereof, note: Alkenes do not directly react with NaOH in this way. This step needs revision to be a relevant demonstration).

Observations:

Methane does not readily react with bromine at room temperature without a catalyst or UV light, while ethene reacts with bromine via an addition reaction to form 1,2-dibromoethane (color change from orange/brown to colorless). Ethene reacts with potassium permanganate (cold, dilute) via an oxidation reaction resulting in a color change from purple to colorless. Alkanes are generally unreactive with hydrochloric acid, while alkenes do not typically react with sodium hydroxide. The phenolphthalein observation is unrelated to the alkene reaction.

Significance:

This experiment (with modifications to steps 1, 4, and 5) demonstrates the different reactivities of alkanes and alkenes. Alkanes are relatively unreactive due to the strong C-C single bonds and lack of polar sites, while alkenes are more reactive due to the presence of the pi bond in the carbon-carbon double bond, allowing for addition reactions. These different reactivities are due to the different hybridization of the carbon atoms: sp3 in alkanes and sp2 in alkenes. The reactions of alkenes are important in various industrial processes such as the production of polymers (addition polymerization).

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