alkanes

Question 1

alkane

Answer 1

Alkanes are the simplest organic compounds made uf carbon
and hydrogen only.

Question 2

examples of alkane

Answer 2

The first member methane (CH4) and its molecule is built of one carbon atom joined to four hydrogen atoms by covalent bonds. Other hydrocarbons of this series: ethane, propane, butane etc, contain two or more carbon atoms linked in
chains by single covalent bonds, the rest of their valencies being
satisfied by union with hydrogen atoms

Question 3

saturated hydrocarbon

Answer 3

Since all the four valencies of the carbon atoms in these compounds are fully satisfied, they can take up no morehydrogen or any other atoms. Thus being ‘fully saturated’ in respect of chemical
combination, these hydrocarbons are classed as Saturated hydro-carbons,

Question 4

paraffin

Answer 4

The saturated hydrocarbons are relatively stable to most
chemical reagents. They react with nitric acid and sulphuric acid only under drastic conditions, while they are not attacked at all by oxidising agents such as potassium, permanganate and chromic acid. On account of their feeble chemical activity they are also called Paraffins

Question 5

general formula of alkanes

Answer 5

CnH2n+2 and their type formula is RH.

Question 6

n vs iso vs neo

Answer 6

the alkanes having straight chanes re named as n-butane, whil branced chain are named as isobutane.

from pentane onwards, if the nomral carbon chain has 2 carbon branches present at C2 we put neo ex” neopentane

Question 7

types of carbon in alkane

Answer 7

primary- bonded to one oher carbon
-seconday- 2
-tertiary- 3
-quaternary- 4al

Question 8

alkyl radical

Answer 8

A group of atoms having unusual valency (repreented by
dash) is called a radical. The radicals derived from alkanes (R-H) by removal of one H atom are called Alkyl radical (R - ).

Question 9

alkanes occurance in nature

Answer 9

The alkanes are distributed in nature in enormous quantities.
Natural gas given off in petroleum regions is a mixture of chiefly
methane and ethane. Lower alkanes are also produced on an
* industrial scale by the dry-distillation of fats and brown coal.

The liquid hydrocarbons are found in almost unlimited quantities in crude petroleum or mineral oil. Natural wax or Ozokerite found in large deposits in petroleum areas and elsewhere is a mixture
of higher solid hydrocarbons.

Question 10

methods of formation

Answer 10

Since the alkanes resemble each other very closely in their
physical properties, they cannot be profitably separated from natural petroleum. Even repeated distillation or crystallisation of the oil
does not yield pure hydrocarbons. Thus for the preparation of
alkanes we have to depend entirely on the synthetic methods.

Question 11

direct synthesis from elements

Answer 11

The lower hydrocarbons, methane and ethane are formed by passing the electric arc
between carbon electrode in an atmosphere of hydrogen. At the
temperature of the arc (1200°) carbon and hydrogen combine
together to form alkanes.

Question 12

1-complete or total synthesis

Answer 12

This method is of special theoretical interest as it forms the first step in the Complete Synthesis or Total Synthesis of any organic compound. By the term ‘complete synthesis’ we mean the building up of a chemical compound starting from elements.

Question 13

2- hydrogenation of unsaturated hydrocarbons

Answer 13

Alkanes are formed by passing a mixture of an unsaturated hydrocarbon and hydrogen over finely divided nickel (or platinum) at 250C to 300 C

Since unsaturated hydrocarbons are obtained in large quantities by the cracking of petroleum, this method is now used for the
technical preparation of alkanes.

Question 14

decarboxyltion of fatty acids

Answer 14

When the sodium salt
of a fatty acid RCOOH, is heated with soda lime (NaOH+CaO), a
molecule of carbon dioxide is split off as carbonate and an alkane obtained.
Obviously, the hydrocarbon obtained by this method contains one carbon atom less than the acid from which It i.s prepared. However, it is a convenient method for the preparation of lower alkanes
in the laboratory.

Question 15

reduction of alkyl halides

Answer 15

When reduced with nascent
hydrogen, alkyl halides are converted to the corresponding alkanes. The common reducing agents employed for the purpose are zinc
and hydrochloric acid; hot hydrogen iodide; and zinc-copper couple
and alcohol.

Question 16

reduction of alcohols, ketones, and fatty acid

Answer 16

Alcohols, ketones and fatty acids when reduced with hot hydriodic
acid and red phosphorus form alkanes
The red phosphorus reacts with iodine produced in the reaction to regenerate HI which is reused.
This method is particularly important for the preparation of
hIgher alkanes from fatty acids which are easily obtained by the
hydrolysis of fats..

Question 17

action of sodium on alkyl halides

Answer 17

Higher alkanes are produced by heating an alkyl halide with
sodium metal in dry ether solution. Two molecules of the alkyl
halide reaot with two atoms of sodium to give alkane with double the
nurrber of carbon utoms present in the former…..
….. long read from text

Question 18

from grignards reagent

Answer 18

Alkylmagnesium halides
(Grignard reagents) when decomposed with water yield pure alkanes.

Question 19

electrolyi of salt of fatty acids

Answer 19

When concentrated solution of sodium or potassium salt of a fatty acid is electrolysed, a higher alkane is obtained.

Question 20

phyical characteritic of alkane

Answer 20

(1) The first four hydrocarbons methane, ethane, propane and
butane are colourless. odourless gases. The next eleven members
(C5H12 to C15H33) are colourless liquids having “benzine” odour. The
higher alkanes are solids with no colour or odour.
(2) Alkanes are the most volatile organic compounds, their
boiling points being the lowest when compared with those of any of their derivatives.
(3) The liquid alkanes are lighter than water.

Question 21

more physical properties

Answer 21

(4) They are almost insoluble in water but dissolve freely in
organic solvents like ether acetone, and carbon tetrachloride.
(5) Their boiling points and specific gravities increase with
rise of molecular weight. Among the lower members, a difference
of CH2 corresponds to an increase of about 30° in the boiling point.
(6) As a rule the boiling points of the hydrocarbons having
branched carbon·chains are lower than those of the isomeric normal hydrocarbons. The increased branching of the chain results in a consequent decrease of the boiling point.

Question 22

chemical characteristics

Answer 22

-stability
-subtitution reaction
-oxidation rxn

Question 23

stability

Answer 23

(i) STABILITY. They are characterised by their stability to
most chemical reagents. The four valencies of the carbon atom being fully satisfied, there is no scope for further .additi~n of hydrogen, halogens, etc. Moreover a carbon-to-carbon linkage is unusually strong and cannot be rupture by even such reagents as potassium
permanganate, chromic acids

Question 24

subtitution reaction

Answer 24

(ii) SUBSTITUTION REACTIONS. The linkage between carbon
and hydrogen is comparatively weak so that the hydrogen atom can be replaced by other atoms or groups giving substitution products.

Question 25

oxidation rxn

Answer 25

tiii) OXIDATIOH REACTIONS. When burnt in air or oxygen, the
alkanes arc decomposed to form carbon dioxide and water. On slow combustion the products are found to cont.ain aldehydes and fatty acids.
As a general rule reactivity of the alkanes increases as one
proceeds from primary, through a secondary to a tertiary hydrogen atom. Thus the H-atoms in isopentane will be attacked in the
order T> S > p

Question 26

halogenation