Preparation of Alkanes Flashcards
Methods for preparation of alkanes
- From unsaturated hydrocarbon
- From alkyl halides
- From carboxylic acids
The unsaturated hydrocarbons (alkenes and alkynes) are converted into alkanes by _________
catalytic hydrogenation
In unsaturated hydrocarbon process, ________ is passed through alkenes or alkynes in the presence of finely divided catalysts
dihydrogen
Dihydrogen is passed through alkenes or alkynes in the presence of finely divided catalysts such as :
Raney Ni, Pt or Pd.
Raney Ni, Pt or Pd metals absorb _________ on their surfaces and activate the hydrogen-hydrogen bond.
dihydrogen gas
_________ and _________ catalyze the reaction at room temperature.
Platinum and palladium
higher temperature (523-573k) and pressure are required with ______ catalysts
nickel
The hydrogenation reaction of unsaturated hydrocarbon using nickel at a temperature of 523-573K is commonly known as _______
Sabatier and Sender’s reaction or reduction.
This method must contain at least two carbon atom.
hydrogenation
_______ (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes.
Alkyl halides
This reaction is known as _______ and is used for the preparation of higher alkanes containing even number of carbon atom
Wurtz reaction
Alkyl halides on treatment with sodium metal in ______ (free from moisture) solution give higher alkanes.
dry ethereal
What are the methods of carboxylic acid
Decarboxylation reaction
Kolbe’s electrolytic method
This process of elimination of carbon dioxide from a carboxylic acid is known as __________
Decarboxylation
Sodium salts of carboxylic acids on heating with soda lime (mixture of sodium hydroxide and calcium oxide) gives alkanes containing one carbon atom less than the carboxylic acid.
Decarboxylation reaction
An aqueous solution of sodium or potassium salt of a carboxylic acid on electrolysis gives alkane containing even number of carbon atoms at the anode.
Kolbe’s electrolytic method
______ are almost non-polar molecules and therefore the molecules are hold only by weak Van der Waals forces.
Alkanes
The ________ depend only upon the size and the structure of the molecule.
weak intermolecular forces
_______ have generally low boiling points because these are non-polar and the molecules are held together only by weak Van der Waals’ forces
Alkanes
Increase in number of carbon atoms, the molecular size increases, magnitude of Van der Waals forces also increases, so boiling points ______
increases
It has been observed that each carbon added to the chain increases the boiling point by _______
20-30 k.
The _______ points of alkanes do not shows regular variation with increase in molecular size.
melting
the alkanes with even number of carbon atoms have _____ melting points because of the symmetrical structures and result in closer packing in the crystal structure as compared to lower alkanes with odd number of carbon atoms.
higher
Value of n = 1-4, the alkanes are _____ under normal conditions
gases
Value of n = 5-18, the alkanes are ______
liquids
Value of n = 18+, the alkanes are ______
solids
Alkanes are soluble in _______
organic solvents
Alkanes are insoluble in _____
water
the solubility generally decreases with increase in ______
molecular mass
______ are lighter than water. The density increase with the increase in the number of the carbon atoms.
Alkanes
The reaction in which an atom or a group of atoms in a molecule is replaced by some other atom or group of atom.
Substitution reaction
Alkanes undergo substitution reaction in which one or more hydrogen atoms are replaced or substituted by different atoms or groups such as _________
halogen atom (Cl, Br or I), nitro group(-NO2) or sulphonic acid (-SO3H) group.
The reaction is initiated by homolysis of chlorine molecule in the presence of light or heat, the Cl-Cl bond is weaker than the C-C and C-H bond and hence, is easiest to break.
Initiation
Chlorine free radicals attacks the methane molecule and takes the reaction in the forward direction by breaking the C-H bond to generate methyl free radical with the formation of H-Cl.
Propagation
The reaction stops after some time due to consumption of reactants and/or due to following side reaction:
Termination
Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat.
Combustion
