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Alkene Reactions

We know that the organic compounds which are made up of carbon and hydrogen are known as hydrocarbons. They have carbon-carbon covalent bonds which can be single or multiple like double or triple covalent bonds. The hydrocarbons that have only carbon-carbon single covalent bonds are known as alkanes. Alkanes are saturated hydrocarbons therefore they are least reactive and usually give substitution reactions. Alkanes are also known as paraffin which stands for ‘least reactivity’. On the contrary, hydrocarbons with the double covalent bond between carbon atoms are known as alkenes.

The general formula of alkenes is $C_{n}H_{2n}$ where ‘n’ indicates the number of carbon atoms. Similarly alkynes are hydrocarbons with at least one triple covalent bond between carbon-carbon atoms. The general formula of alkynes is $C_{n}H_{2n-2}$; here ‘n’ represents the number of carbon atoms. Both alkenes and alkynes are unsaturated hydrocarbons. The unsaturation in hydrocarbons makes them more reactive compare to saturated hydrocarbons. This is due to the presence of pi bonds in multiple bonds.

Number of carbon atoms Formula (CnH2n)
Name/Isomer
1
Does not exit -
2 CH2=CH2
Ethene
3
CH3-CH=CH2
Propene
4
CH3-CH=CH=CH3
CH2=CH-CH2-CH3
2-Butene, 1-Butene
5
CH2=CH-CH2-CH2-CH3
CH3-CH=CH-CH2-CH3
1-Pentene, 2-Pentene
5
CH2=CH-CH2-CH2-CH2-CH3
CH3-CH=CH-CH2-CH2-CH3
CH3-CH2-CH=CH-CH2-CH3
1-Hexene, 2-Hexene, 3-Hexene

Related Calculators
Chemical Reaction Calculator Redox Reaction Calculator
 

Physical Properties of Alkenes

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Properties of compound which can be observed or measured without changing the composition are termed as physical properties. Physical properties of substances are basically a tool used for observing and describing a matter. For example, appearance and texture of compound, color, odor, melting point and boiling point, density, solubility, polarity, and surface tension, viscosity etc.

Alkenes are unsaturated hydrocarbons; hence show different physical properties compare to saturated hydrocarbons like alkane and show resemblance with another type of unsaturated hydrocarbons that is alkyne with triple bond. Let’s discuss some of the physical properties of alkenes like physical state, density, polarity, melting point and boiling point.
Other member of homologous series of alkenes is as follows.

Physical State

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Physical State of Alkenes has Given Below:-
  1. Alkenes can exist in all the three physical states; gases, liquids, and solids at room temperature.
  2. At room temperature, lower alkenes, first three alkenes are colorless gases, while next fourteen members those comprise of five to fifteen carbons atoms are colorless liquids.
  3. The higher alkenes are colorless solids.
  4. Except ethene all other alkenes are colorless and odorless.
  5. Ethene has a faint sweet smell. Because of non-polar nature of alkenes, they are insoluble in water but soluble in non-polar solvents like benzene, carbon tetrachloride and petroleum ether, etc.

Alkenes have densities less than 1 g/ml and are therefore less dense than water, hence form separate layer above water.

Alkene
Structure
Density (g/ml)
Ethene CH2=CH2 0.6128
Propene
CH3-CH=CH2
0.6142
1-Butene
CH2=CH-CH2-CH3
0.6356
Cis-2-Butene
CH3-CH=CH-CH3 0.641
Trans-2-Butene
CH3-CH=CH-CH3 0.626
1-Pentene
CH2=CH-CH2-CH2-CH3 0.64
Cis-2-Pentene
CH3-CH=CH-CH2-CH3 0.65
Trans-2-Pentene
CH3-CH=CH-CH2-CH3 0.649
1-Hexene CH2=CH-CH2-CH2-CH2-CH3 0.673

As the molecular mass of alkenes increases, density increases.

Alkene Density Graph

The position of double bond in an alkene does not affect much on the value of density. However density of trans-isomer is less than cis-isomers of alkenes. → Read More

Boiling Points of Alkenes

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In liquid state molecules are packed closely together with weak intermolecular force of attraction, hence can be move randomly in system. When we supply heat to the liquid, temperature of system increases which contributes in increment in kinetic energy of molecules. Hence the molecule motion increases and the forces of attraction between the molecules decrease at high temperature.
Ultimately the forces of attraction between the molecules are disrupted up to the extent that the molecules get free and become a gas.
  • The temperature at which liquid turns into a gas is called as boiling point.
  • In case of alkene, their boiling point shows similarity with alkanes having same number of carbon atom.
  • As in both hydrocarbons have only weak van der wall dispersion force of attraction.
  • The boiling points of alkenes increase with an increase in their molecular masses.
  • The intermolecular attractions become stronger with an increase in the size of the molecules.
  • Cis-isomers are polar molecules and have higher boiling points when compared to the trans-isomers.
  • As the polar molecules attracted by the opposite charges which required some amount of energy to cleaved.
  • Hence greater the polarity, more the force of attraction leads to high boiling points.

Alkene
Structure Boiling point (oC)
Ethene CH2=CH2 -104
Propene
CH3-CH=CH2 -47
1-Butene
CH2=CH-CH2-CH3 -6.5
Isobutene
CH2=CH(CH3)2 -7
Trans-2-Butene
CH3-CH=CH-CH3 0.9
Cis-2-Butene
CH3-CH=CH-CH3 3.7
1-Pentene
CH2=CH-CH2-CH2-CH3 30
Trans-2-Pentene
CH3-CH=CH-CH2-CH3 36
Cis-2-Pentene
CH3-CH=CH-CH2-CH3 37
1-Hexene
CH2=CH-CH2-CH2-CH2-CH3 63.5
1-Heptene
CH2=CH-CH2-CH2-CH2-CH2-CH3 115
1-Octene
CH2=CH-CH2-CH2-CH2-CH2-CH2-CH3 122.5
3-Octene
CH3-CH2-CH=CH-CH2-CH2-CH2-CH3 122
3-Nonene
CH3-CH2-CH=CH-CH2-CH2-CH2-CH2-CH3 147
5-Decene
CH3-CH2-CH2-CH2-CH=CH-CH2-CH2-CH2-CH3 170
3-Methyl-1-butene
CH2=CH-CH(CH3)CH3 25
2-Methyl-2-butene
CH3-C(CH3)=CH-CH3 39
2,3-Dimethyl-2-butene
CH3-C(CH3)=C(CH3)CH3 73

Melting Points of Alkenes

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Melting point of any organic compound depends on the packing arrangement of molecules in solid state. A symmetrical molecule packed nicely in system and hence melting point increases.
Just like boiling point, melting points of alkenes also increase with an increase in the molecular mass.Because of symmetrical nature of trans-isomers, they show higher melting point compare to cis isomers. This is because of the two terminal carbon atoms on opposite sides that lead to a more close packing of molecules as compared to cis alkene.For example; symmetrical alkene like trans-2-pentene melts at -135°C while the melting point of cis-2-pentene is -180°C.

Cis Trans Isomers of Pentene
The melting points of some alkenes are as follow.

Compound
Structure
Melting point(oC)
Ethene CH2=CH2 -169
Propene
CH3-CH=CH2
-185
Cis-2-butene
CH3-CH=CH-CH3
-138
Trans-2-butene
CH3-CH=CH-CH3 -105.5
1-Butene
CH2=CH-CH2-CH3
-185
3-Methyl-1-butene
CH2=CH-CH(CH3)CH3 -135
2-Methyl-2-butene
CH3-C(CH3)=CH-CH3 -123
2,3-Dimethyl-2-butene
CH3-C(CH3)=C(CH3)-CH3 -74
Isobutylene
CH2=CH(CH3)2 -141
1-Pentene
CH2=CH-CH2-CH2-CH3 -165
Trans-2-pentene
CH3-CH=CH-CH2-CH3 -135
Cis-2-pentene
CH3-CH=CH-CH2-CH3 -180
1-Hexene
CH2=CH-CH2-CH2-CH2-CH3 -138
1-Heptene
CH2=CH-CH2-CH2-CH2-CH2-CH3
-119
3-Octene
CH3-CH2-CH=CH-CH2-CH2-CH2-CH3 -101.9
3-Nonene
CH3-CH2-CH=CH-CH2-CH2-CH2-CH2-CH3 -81.4
5-Decene
CH3-CH2-CH2-CH2-CH=CH-CH2-CH2-CH2-CH3 -66.3

The polarity of organic compounds depends on their chemical structure, substitution and presence of other functional groups. In alkenes the double bonded carbon is sp2 hybridized which is more electronegative compare to sp3 hybridized carbon atom in an alkane, and hence in alkene the carbon atom is much more electron-withdrawing than the sp3 hybridize orbitals and creates a weak dipole in alkenes.In substituted alkenes the dipole of molecules depends on the geometry of molecule. In trans-isomer, same groups located at opposite position, hence they cancel the dipole of each other out and overall molecule becomes non-polar. However, cis-isomers of alkenes are polar in nature. For example: the cis-2-Butene shows a dipole moment of 0.33 D, while the trans-isomer shows zero dipole moment because of cancellation of the electronic effects.

Because of dipole moment, cis and trans isomers of alkene can be distinguish easily as dipole moment contribute in other physical properties also like boiling point. Some alkenes like propene and 1-butene are weakly polar with dipole moment 0.35D because of their unsymmetrical geometry.
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