Properties of Alkanes/ Alkenes/ Alkynes (10th Carbon and its compounds) CBSE_NCERT Class Xth

10th Carbon and its compounds_ CBSE_NCERT Class Xth 

(a) Alkanes are gas (C₁to C₄) ; liquid (C₅to C₁₇) and rest are solid at room temperature.

(b) The melting and boiling point of Alkanes increases with increase in number  of C  – atom in Alkanes .

Alkane	Formula	Boiling point [°C]	Melting point [°C]	State (at 20 °C)
Methane	CH4	-162	-182	gas
Ethane	C2H6	-89	-183	gas
Propane	C3H8	-42	-188	gas
Butane	C4H10	0	-138	gas
Pentane	C5H12	36	-130	liquid
Hexane	C6H14	69	-95	liquid
Heptane	C7H16	98	-91	liquid
Octane	C8H18	126	-57	liquid
Nonane	C9H20	151	-54	liquid
Decane	C10H22	174	-30	liquid
Undecane	C11H24	196	-26	liquid
Dodecane	C12H26	216	-10	liquid
Hexadecane	C16H34	281	18	liquid
Icosane	C20H42	343	37	solid
Triacontane	C30H62	450	66	solid
Tetracontane	C40H82	525	82	solid
Pentacontane	C50H102	575	91	solid
Hexacontane	C60H122	625	100	solid
Source: http://en.wikipedia.org/wiki/Alkane

Reason: 
Alkanes experience inter-molecular van der Waals forces. Stronger inter-molecular van der Waals forces give rise to greater boiling points of alkanes.

A straight-chain alkane will have a boiling point higher than a branched-chain alkane due to the greater surface area in contact.  

Note:  In Chemistry the Van der Waals forces include attractions and repulsions between atoms, molecules, and surfaces, as well as other intermolecular forces.

 (c) The density of the alkanes usually increases with increasing number of carbon atoms

Alkane	Formula	Density
Pentane	C5H12	0.626 (liquid)
Hexane	C6H14	0.659 (liquid)
Heptane	C7H16	0.684 (liquid)
Octane	C8H18	0.703 (liquid)
Nonane	C9H20	0.718 (liquid)

(d) Alkanes are generally insoluble in polar compound like water but dissolve in organic solvents (Non polar compound) like benzene. The liquid alkanes are good solvents for many other covalent compounds.

Reason: Alkanes do not conduct electricity, nor are they polarized by electricity. For this reason they do not form hydrogen bonds and are insoluble in polar solvents such as water.

(e) Alkanes are stable and less reactive than alkenes

Reason: This is because saturated hydrocarbons contain only single bonds which are very stable and difficult to break. On the other hand, unsaturated hydrocarbons contain pie bonds, which can be easily broken as they are more strained.

(f) Alkanes are generally good combustible material due presence of the good percentage of Hydrogen.

Since Alkanes burn with non smoky and non sooty flame due to presence of the good percentage of Hydrogen, It is used as a fuel like LPG and CNG

2CH₄   +          2O₂                 →        CO₂    +          H₂O    +          Heat

2C₂H₆  +          7O₂                  →        4CO₂   +          6H₂O  +          Heat

However, if the supply of air or oxygen is not sufficient for complete combustion, carbon monoxide is formed. Carbon monoxide (CO) is highly poisonous.

2CH₄              +          3O₂      →                    2CO    +          4H₂O

2C₄H₁₀           +         9O₂      →                    8CO    +         10H₂O

(g) Alkanes do not undergo addition reaction due to strong van der wall force.

Unlike alkane, generally  alkene Undergo  addition reaction with hydrogen gas in the presence of Polladium catalyst  and gives saturated hydrocarbon

				Pd
CH2     =    CH2	+	H2	--------------->			CH3    -   CH3

ThIs addition reaction of hydrogen is helpful in converting vegetable Oil into saturated fat like vanaspati  gee . This reaction is commonly called Hydrogenation of Oil.

Note: Butter contain saturated compound where as cooking oil contain unsaturated compound. If Alkaline KMno₄ added to both, pink color of KMno₄ disappear in cooking oil but remain in pink in butter.

				Ni
Vegetable Oil	+	H2	------------>			CH3    -   CH3

(h)  Alkanes undergo addition reaction because hydrogen attached to carbon easily replaced by atom more reactive than hydrogen like halo atom Cl,Br ,I etc.

			Sunlight
CH4	+	Cl2	------------>	CH3Cl	+	HCl
			Sunlight
CH4Cl	+	Cl2	------------>	CH2Cl2	+	HCl
			Sunlight
CH2Cl2	+	Cl2	------------>	CHCl3	+	HCl
			Sunlight
CHCl3	+	Cl2	------------>	CH3Cl	+	HCl

If chlorine present in excess , then reaction does not stopin between but keep on reacting till it form carbon tetra chloride

Bromine and chlorine react with alkene to form vicinal dihalides (iodine does not undergo addition reaction under normal conditions).

			CCl4
CH2 = CH2	+	Cl2	----------->	CH2	-	CH2
				I		I
				Br		Br
Ethene				1,2-Dibromoethane

CH2 = CH2	+	Cl2	-------->	CH3	-	CH	-	CH2
						I		I
						Cl		Cl
Ethene				1,2-Dibropropane

Note: In Alkanes, the four valencies of carbon atom are saturated and they have stable s (sigma) bonds. Any nuleophile comes, one hydrogen atom is replaced to accommodate the nucleophile.

A nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction.

e.g. CH₄ + Cl₂   --------> CH₃Cl + HCl

In case of Alkene or Alkyne, Carbon atom has double or triple bonds. In this, one is stable s (sigma)  bond and the rests are unstable(or weak) p  bonds. When nucleophile approches to the carbon atom that has double or triple bonds, they easily break to accommodate it.

e.g., H₂C=CH₂ + Cl₂  --------> H₂ClC-CH₂Cl.

(i) Cracking (or pyrolysis):  Higher alkanes undergo thermal decomposition to give lower alkanes. This process is called pyrolysis or cracking. In this process, vapour of higher alkanes is passed through a hot metal tube (500⁰C – 700⁰C). Propane on cracking gives,

		D				C3H6 + H2
C3H8	------------>
						CH4 + C2H4
Cracking of hexane gives butane and ethane.
			D
C6H14	------------->					C4H10 + C2H4

Q. What's the difference between cracking and pyrolysis?

Pyrolysis : The decomposition of a compound on heating in absence of air is known as pyrolysis.

Cracking: is the breakdown of large organic compounds by use of a catalyst and low temperature to form fewer different compounds

Q.  In cracking will the decomposed hydrocarbon always be a saturated and a unsaturated hydrocarbons? Can they both be either unsaturated or saturated hydrocarbons?

Cracking is defined as the process of breaking or decomposition of large hydrocarbons into smaller compounds. This process is widely used in the petroleum industry. Usually, we get a mixture of alkane and alkenes by the process of cracking of hydrcarbons. This is because the bonds in the hydrocarbons can be cleared in any way. As a result, we will not get alkane and alkene exclusively but a mixture of both.

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