Plus One Chemistry Notes Chapter 13 Hydrocarbons is part of Plus One Chemistry Notes. Here we have given Kerala Plus One Chemistry Notes Chapter 13 Hydrocarbons.
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Kerala Plus One Chemistry Notes Chapter 13 Hydrocarbons
The compounds formed by carbon and hydrogen are called hydrocarbons.
Classification at Hydrocarbons
Depending upon the types of carbon-carbon bonds present, they can be classified into 3 main categories:
- Saturated hydrocarbons
Alkanes (C-C), single bond
- Unsaturated hydrocarbons
Alkenes (C=C), double bonds
Alkynes (C = C), triple bonds
- Aromatic hydrocarbons or Arenes
Aromatic hydrocarbons contain one or more benzene rings which is a 6 membered ring of carbon atoms linked to each other with alternate single and double bonds.
Alkanes or Paraffins (Cn H2n+2)
Alkanes are saturated open chain hydrocarbons containing carbon-carbon single bonds,
eg., CH4 (methane)
- C1 to C4 are gases, C5 to C17 are liquids, and C18 to above are solid.
- Boiling point ex-molecular mass a van der Waals force of attraction.
- The branched chain isomer has lower boiling point than the corresponding n-alkane due to surface area.
Alkanes are generally inert towards acids, bases, oxidising and reducing agents. However, they undergo the following reactions under certain conditions.
3. Controlled oxidation.
Alkanes resist oxidation in ordinary condition. Alkanes on oxidation with strong oxidising agent (KMnO4) give corresponding alcohols.
Alkanes on heating with air or O2 at high pressure with the presence of a catalyst give a variety of oxidation products.
The straight chain alkanes when heated with AICI3 in presence of dry HCl at 300 °C give branched chain isomers.
Alkanes having six or more carbon atoms on heating to 773 K at 10-20 atmospheric pressure in presence of Cr2O3 or V2O5 or MO2O3 catalyst give aromatic compounds. This process is known as aromatization.
6. Pyrolysis or Cracking.
The process of decomposition of less votatile higher hydrocarbons from petroleum into more votatile lesser hydrocarbons with the application of strong heat is known as cracking.
What Is the number of a and K bonds In
N ≡ C-CH ≡ CH-C ≡ N
There are 7σ bonds and 5 πbonds
Thermal cracking and Catalystic cracking
Thermal cracking is carried out at high temperature and pressure. Catalytic cracking is carried out at low temperature and pressure in presence of catalyst such as silica and alumina. The quality of gasoline obtained by catalytic cracking is superior than thermal cracking.
Conformations in Hydrocarbons (Conformers or Rotamers):
Spatial arrangement of the atoms which can be converted in to one another by rotation around a C-C single bond are called conformation or eonformers or rotamers.
Alkane can have infinite number of conformations by rotation around C-C single bonds. It is hindered by a small energy barrier of T-20 kJ/mol due to weak repulsive interaction between the adjacent bonds. Such a type of repulsive interaction is called Torsional strain.
Conformation of Ethane
The rotation in Ethane results into infinite number of spatial arrangements of hydrogen atoms attched to one carbon atom with respect to the hydrogen atoms attached to the other carbon atom. There are called conformational isomers.
Two conformation of Ethane:
- Eclipsed conformations
- Staggered conformations
Any other intermediate conformation is called a skew or gauche conformations.
Draw the Newman’s projection formula of the staggered form of 1,2-dichiorethane
Alkenes are unsaturated hydrocarbons containing at least one double bond.
- The first three members are gases, the next fourteen are liquids and the higher ones are solids.
- Ethene is a colourless gas with a faint sweet smell. All other alkenes are colourless and odourless.
- Insoluble in water but fairly soluble in nonpolar solvents like benzene,petroleum ether.
- They show a regular increase in bailing point with increase in size ie., every -CH2 group added increases boiling point.
- Like alkanes, straight chain alkenes have higher boiling point than isomeric branched chain compounds.
1. Addition of Hydrogen.
H addition to alkenes form alkanes in presence of finely divided nickel.
2. Addition of Halogen.
Halogens added to alkene to form vicinal dihalides
3. Addition of Hydrogen halides.
Addition of HBr to symmetrical alkenes (ethene).
This rule states that when the addition of an unsymmetrical reagent across an unsymmetrical double bond take place, the +ve part of the adding reagent goes to the carbon atom with the greater number of hydrogen atoms.
4. Addition of water (Hydration).
Alkenes react with water in presence of H2SO4to form alcohols (hydration).
Alkenes on oxidation with Baeyer’s reagent (cold, dilute, aqueous solution of KMnO4 form vicinal glycols. Decolourisation of KMnO4 solution is used as a test for unsaturation.
The process by which monomers (small molecules or alkenes) are transformed in to a polymer is called polymerisation.
Alkynes are unsaturated hydrocarbons contain at least one triple bond between two carbon atoms. General formula is CnH2n2. eg., acetylene or Ethyne C2H2 or CH = CH.
C2 to C3 are gases. C4 to C8 are liquids and higher members are gases. Alkynes are weakly polar and soluble in organic solvent. Their m.p, b.p and density increase with increase in molecular mass.
Acidic character of Alkyne. Due to maximum percentage of s character (50%), the sp hybridised orbital of carbon atoms have highest electronegativity. Hence attract the shared electron pair of C-H bond of ethyne to a greater extent than of sp2 in ethene and sp3 in ethane. As a result in ethyne, hydrogen atom can be liberated as protons more easily.
Molecular orbital concept of benzene.
Each carbon atom in benzene is sp2 hybrised 6 C-H sigma bonds are present. It has a flat regular hexagonal structure with bond angle 120°. Six unhybridised Pz orbitals overlap with adjacent carbon atoms to form n bonds. It is the delocalisation of π electron cloud that makes the benzene molecule more stable.
Huckel rule or (4n+2) rule.
This rule states that for a compound to exhibit aromatic character, it should have a conjugated, planar cyclic system containing 4n+2 (where n = 1,2, 3,…) delocalised 7i electons forming a cyclic cloud above and below the plane of the molecule. This is known as Huckel rule of (4n+2) n electrons.
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