Hydrocarbons

Hydrocarbons PDF Notes, Important Questions and Synopsis

SYNOPSIS

Alkanes: General formula: C_nH_{_2n+2}

Preparation of alkanes
Chemical properties of alkanes

Aromatisation:

$begin mathsize 12px style straight a. space straight R minus straight H rightwards arrow from heat to Al subscript straight 2 straight O subscript straight 3. Cr subscript straight 2 straight O subscript straight 3 of thin space Cycloalkane thin space rightwards arrow from blank to blank of thin space Benzene + its thin space derivative end style$
Cracking (pyrolysis):
$begin mathsize 12px style text a. Decane end text rightwards arrow from heat to Conc. straight H subscript straight 2 SO subscript straight 4 of thin space Heptane + Propene + Hexane plus Butene plus Pentane plus Pentene plus. .. end style$

_{_{Conformations of ethane:}}

Conformations →	Eclipsed	Staggered	Skew
	Conformation in which hydrogen atoms attached to two carbon atoms are as close to each other as possible	Conformation in which hydrogen atoms are as farther away as possible	Any other intermediate conformation in between eclipsed and sawhorse
Sawhorse projection
Newman projection		[RKR1]

Geometrical isomers of alkenes

Trans-isomer is non-polar and has zero dipole moment where a cis-isomer is polar.
Cis-isomer has higher boiling points compared to trans-isomer because it is polar.
In case of solids, cis-isomer has a lower melting point than trans-isomer. This is because the trans-isomer is symmetrical and fits well into the crystal lattice.
. Preparation of alkenes

Chemical properties of alkenes

Test for Unsaturation: There are two tests to know whether a compound is unsaturated or not
1. Bromine water test: When an unsaturated compound is reacted with bromine solution in carbon tetrachloride, the reddish-orange colour of bromine solution in carbon tetrachloride is discharged.
2. Baeyer’s reagent test: When an unsaturated compound is reacted with cold, dilute and aqueous solution of potassium permanganate (Baeyer’s reagent), the pink colour of KMnO4 solution is discharged.

Preparation of alkynes

Acidic nature of alkynes: Hydrogen atoms of ethyne attached to triple bonded carbon atom are acidic in nature.
Example:

$begin mathsize 12px style table attributes columnalign left end attributes row cell HC identical to CH text end text plus text end text Na text end text rightwards arrow from blank to blank of text end text stack HC identical to straight C to the power of minus Na to the power of plus with table attributes columnalign left end attributes row Monosodium row cell text end text ethynide end cell end table below text end text plus text end text 1 half text end text straight H subscript 2 end cell row cell HC identical to straight C to the power of minus Na to the power of plus text end text plus text end text Na text end text rightwards arrow from blank to blank of text end text stack Na to the power of plus straight C to the power of minus identical to straight C to the power of minus Na to the power of plus with table attributes columnalign left end attributes row disodium row cell text end text ethynide end cell end table below text end text plus text end text 1 half text end text straight H subscript 2 end cell end table end style$

Reason:

	Alkyne	Alkene	Alkane
Hybridisation of carbon to which H atom is attached	sp	sp²	sp³
Percentage s character of carbon	50% (maximum)	66.6%	25% (minimum)
Electronegativity of carbon atom	Highest	Less than alkyne and more than alkane	Lowest
Extent of attraction of hydrogen atoms of C-H bonds towards C	Highest	Less than alkyne and more than alkane	Lowest
Ease of liberation of H atoms as protons	Highest	Less than alkyne and more than alkane	Lowest
Acidic character	Highest	Less than alkyne and more than alkane	Lowest

Chemical properties of alkynes

Benzene:
Conditions for aromaticity

Planarity: The ring should be planar. In simple words, all the carbons of the ring should be sp2 hybridised.
Complete delocalisation of the p electrons in the ring: The p electrons should be completely delocalized. In simple words, in the ring alternate single and double bonds should exist.
Hückel Rule: Presence of (4n + 2) p electrons in the ring where n is an integer (n = 0, 1, 2, . . .).

Chemical properties of benzene

Directive influence of a functional group in monosubstituted benzene

Ortho–para directing groups: Groups which direct the incoming group to
ortho and para positions are called ortho–para directing groups.
Activating groups: Groups which increase the electron density at ortho and para positions of the benzene ring.
Examples: –NH₂, –NH_R, –NHCOCH₃, –OCH₃, –CH₃ and –C₂H₅.Deactivating groups: Groups which decrease the electron
density at ortho and para positions of the benzene ring.
Example: –X.
Meta directing groups: The groups which direct the incoming group to meta position are called meta directing groups.
Examples: –NO₂, –CN, –CHO, –COR, –COOH, –COOR and –SO₃H.