Difference Between Alkanes and Alkenes

Key Difference – Alkanes vs Alkenes
 

Alkanes and Alkenes are two types of hydrocarbon families which contain carbon and hydrogen in their molecular structure. The key difference between Alkanes and Alkenes is their chemical structure; alkanes are saturated hydrocarbons with the general molecular formula of CnH2n+2 and alkenes are said to be an unsaturated hydrocarbon group since they contain a double bond between two carbon atoms. They have the general molecular formula of CnH2n. 

What are Alkanes?

Alkanes contain only single bonds between Carbon and hydrogen atoms (C-C bonds and C-H bonds). Therefore, they are called “saturated hydrocarbons”. According to the orbital hybridization model, all the carbon atoms in Alkenes have the SP3 hybridization. They form sigma bonds with Hydrogen atoms, and the resulting molecule has the geometry of a tetrahedron. Alkanes can be sub-divided into two groups according to their molecular arrangements; acyclic alkanes (CnH2n.+2) and cyclic alkanes (CnH2n).Difference Between Alkanes and Alkenes

What are Alkenes?

Alkenes are the hydrocarbons, containing a Carbon-Carbon (C=C) double bond. “Olefins” is the old name used to refer to the alkene family. The smallest member of this family is ethane (C2H4); it was called olefiant gas (In Latin: ‘oleum’ means ‘oil’ + ‘facere’ means ‘to make’) in early days. This is because the reaction between C2H4 and Chlorine gives C2H2Cl2, oil.Key Difference - Alkanes vs Alkenes

What is the difference between Alkanes and Alkenes?

Chemical Structure of Alkanes and Alkenes

Alkanes: Alkanes have the general molecular formula CnH2n+2. Methane (CH4) is the smallest alkane.

Name Chemical formula Acyclic structure
Methane CH4 CH4
Ethane C2H6 CH3CH3
Propane C3H8 CH3CH2CH3
Butane C4H10 CH3CH2CH2CH3
Pentane C5H12 CH3CH2CH2CH2CH3
Hexane C6H14 CH3CH2CH2 CH2CH2CH3
Heptane C7H16 CH3CH2CH2CH2CH2CH2CH3
Octane C8H18 CH3 CH3CH2CH2CH2CH2CH3CH3

Alkenes: Alkenes have the general chemical formula of CnH2n. Alkenes are considered to be unsaturated hydrocarbons since they do not contain the maximum number of Hydrogen atoms that can be owned by a hydrocarbon molecule.

Name Chemical formula Structure
Ethene C2H4 CH2 = CH2
Propene C3H6 CH3CH = CH2
Butene C4H8 CH2=CHCH2CH3, CH3CH=CHCH3
Pentene C5H10 CH2=CHCH2CH2CH3, CH3CH=CHCH2CH3
Hexene C6H12 CH2=CHCH2 CH2CH2CH3CH3CH=CHCH2CH2CH3

CH3CH2CH=CHCH2CH3

Heptene C7H14 CH=CHCH2CH2CH2CH2CH3CH3CH=CH2CH2CH2CH2CH3

Chemical Properties of Alkanes and Alkenes

Alkanes:

Reactivity:

Alkanes are inert to many chemical reagents. This is because Carbon–Carbon (C-C) and Carbon – Hydrogen (C-H) bonds are quite strong since Carbon and Hydrogen atoms have nearly the same electronegativity values. Therefore, it is very difficult to break their bonds, unless they are heated to fairly high temperatures.

Combustion:

Alkanes can readily burn in the air. The reaction between Alkanes with excess Oxygen is called “combustion”. In this reaction, alkanes convert to Carbon dioxide (CO2) and water.

CnH2n              +          (n + n/2) O2             →            n CO2                   +                 nH2O

C4H10               +         13/2 O2                              →           4 CO2                   +          5H2O

Butane                        Oxygen                               Carbon Dioxide          Water

The combustion reactions are exothermic reactions (they give off heat). As a result, alkanes are used as a source of energy.

Alkenes:

Reactivity:

Alkenes react with Hydrogen in the presence of a finely divided metal catalyst to form the corresponding alkane. The rate of the reaction is very low without a catalyst.Difference Between Alkanes and Alkenes - 01

Catalytic hydrogenation is used in food industry to convert liquid vegetable oils to semi-solid fat in making margarine and solid cooking fat.

Physical Properties of Alkanes and Alkenes

Forms

Alkanes: Alkanes exist as gases, liquids and solids. Methane, ethane, propane and butane are gases at room temperature. The unbranched structures of hexane, pentane and heptane are liquids. Alkanes which have a higher molecular weight are solids.

CH4 to C4H10  are  gases

C5H12  to C17H36  are liquids, and 

Alkanes with higher molecular weight are soft solids

Alkenes: Alkenes show similar physical properties of the corresponding Alkane. Alkenes which have lower molecular weights (C2H4 toC4H8) are gases at room temperature and atmospheric pressure. Alkenes having a higher molecular weight are solids.

Solubility:

Alkanes: Alkanes do not dissolve in water. They are dissolved in non-polar or weakly polar organic solvents.

Alkenes: Alkenes are relatively polar molecules due to the C=C bond; therefore, they are soluble in non-polar solvents or solvents of low polarity. Water is a polar molecule and alkenes are slightly soluble in water.

Density:

Alkanes: The densities of Alkanes are lower than the density of water. Their density value is nearly 0.7 g mL-1, considering the density of water as 1.0 g mL-1.

Alkenes: The densities of Alkenes are lower than the density of water.

Boiling points:

Alkanes: The boiling point of unbranched alkanes smoothly increases as the number of Carbon atoms and the molecular weight are increasing. In general, branched alkanes have lower boiling points compared to the unbranched alkanes, having the same number of Carbon atoms.

Difference Between Alkanes and Alkenes - 02

Alkenes: Boiling points are similar to the corresponding alkanes with a small variation.

References:
 
CliffsNotes. (n.d.). Retrieved July 06, 2016, from here. 
Physical Properties of Alkanes. (2013). Retrieved July 06, 2016, from here 
Zum Directory-modus. (n.d.). Retrieved July 06, 2016, from here
Chemical properties of the Alkanes : Wizznotes.com- Free GCSE and CXC: Tutorials, Past Papers and Quizzes. (n.d.). Retrieved July 06, 2016, from here 
Polarity. (n.d.). Retrieved July 06, 2016, from here
Dipole moments. (2013). Retrieved July 06, 2016, from here
Structural isomer. (n.d.). Retrieved July 06, 2016, from here
An introduction to alkenes. (n.d.). Retrieved July 06, 2016, from here
Image Courtesy:
“Alkene names” By Chris Evans – (CC0) via Commons Wikimedia
“Saturated C4 hydrocarbons ball-and-stick” By Fvasconcellos – Own work (Public Domain) via Commons Wikimedia