DocScanner Chemistry Past Paper PDF

## chhaya Chemistry (XI) Part-II ### Types of H-atoms of n-pentane, isopentane and neopentane by a methyl group * CH3CH2CH2CH2CH2CH3 * Hexane * CH3CHCH2CH2CH3 * CH3 * 2-methylpentane * CH3CH2 CH CH2CH3 * CH3 * 3-methylpentane * CH3-C-CH2-CH3 * CH3...

## chhaya Chemistry (XI) Part-II ### Types of H-atoms of n-pentane, isopentane and neopentane by a methyl group * CH3CH2CH2CH2CH2CH3 * Hexane * CH3CHCH2CH2CH3 * CH3 * 2-methylpentane * CH3CH2 CH CH2CH3 * CH3 * 3-methylpentane * CH3-C-CH2-CH3 * CH3 * 2,2-dimethylbutane * CH3-CH-CH-CH3 * CH3 CH3 * 2,3-dimethylbutane ### Warm up Exercise 1. What type of isomerism is exhibited by alkanes? 2. Write the structures and trivial names of the alkanes of formula C5H12. 3. How many chain isomers will be obtained on replacement of different H-atoms of n-pentane? Write their structures and IUPAC names. ### 13.4.4 Conformational isomerism in alkanes #### Definition Electron distribution of the sigma molecular orbital of a C-C bond is cylindrically symmetrical around the internuclear axis and as this is not disturbed due to rotation about its axis, free rotation about the C-C single bond is possible. Infinite number of spatial arrangements of atoms which result through rotation about a single bond are called conformations or conformational isomers or rotational isomers or simply conformers or rotamers and the phenomenon is called conformational isomerism. The difference in potential energy between the most stable conformation and the conformation under consideration is called the conformational energy of the given conformation. It is to be noted that the rotation around a C-C single bond is not completely free. It is hindered by a very small energy barrier of 1-20 kJ·mol−1 due to very weak repulsive interaction between the electron clouds of different o-bonds. Such repulsive interaction is called torsional strain. Conformations are three-dimensional. These are generally represented in paper by three projection formulae: flying wedge formula, sawhorse projection formula and Newman projection formula. ### Conformations of ethane: A molecule of ethane (CH3-CH3) contains a carbon-carbon single bond (σ bond) and each carbon atom is attached to three hydrogen atoms. The two -CH3 groups can rotate freely around the C-C bond axis. Rotation of one carbon atom keeping the other fixed results into infinite number of spatial arrangements of hydrogen atoms attached to the rotating carbon atom with respect to the hydrogen atoms attached to fixed carbon atom. These are called conformational isomers of conformations or conformers. Thus, there are infinite number of conformations of ethane. However, there are two extreme cases. The conformation in which the hydrogen atoms attached to two carbons are as close together as possible, i.e., in which the dihedral angle between two nearest C-H bonds of two -CH3 groups is zero, is called the **eclipsed conformation**. The conformation in which the hydrogen atoms are as far apart as possible, i.e., the dihedral angle between two C-H bonds is 60° is called the **staggered conformation**. The eclipsed conformation suffers from maximum torsional strain whereas in staggered conformation this strain is minimum. So, the eclipsed conformation is much less stable than the staggered conformation. Any other intermediate conformation i.e., the conformation in which the dihedral angle is between 0-60°, is called the **skew conformation**. Its stability is in between the two extreme conformations. Therefore, the order of stability of these three conformations is: staggered > skew > eclipsed. It is to be noted that in all these conformations, the bond angles and the bond lengths remain the same. Saturated hydrocarbons containing more than two carbon atoms have different conformations. However, as there is only one carbon atom in methane, it does not exist in the above mentioned conformations. The eclipsed and the staggered conformations of ethane can be represented by flying wedge formula, sawhorse projection formula and Newman projection formula as follows: i] **Flying Wedge Formula**: In this representation, the two bonds attached to a carbon atom are shown in the plane of the paper and of the other two, one is shown above the plane and another below the plane. The bonds which are in the plane are shown by normal lines (-) but the bond above the plane is shown by solid wedge (-) and the bond below the plane is shown by hashed wedge (…). ii] **Sawhorse Projection Formula**: In this projection, the molecule is viewed along the molecular axis. It is then projected on paper by drawing the central C-C bond as a somewhat elongated line. Upper end of the line is slightly tilted towards right hand side. The front carbon is shown at the lower end of the line, whereas the rear carbon is shown at the upper end. Each carbon has three lines attached to it corresponding to three H -atoms. The lines are inclined at 120° angle to each other. iii] **Newman Projection Formula**: In this projection, the molecule is viewed along the C-C bond. The C-atom nearer to the eye of the viewer (i.e., the front carbon) is represented by a point and the three H-atoms attached to the front C-atom are shown by the three lines drawn at an angle of 120° to each other. The C-atom situated farther from the eye of the viewer (i.e., the rear carbon) is represented by a circle and the three hydrogen atoms attached to it are represented by three shorter lines drawn at an angle of 120° to each other. ### Conformations of Propane (CH3-CH2-CH3): In propane molecule, both C₁-C₂ & C₂-C₃ bonds are equivalent. An infinite number of conformations of propane can be obtained as a result of rotation about the C₁-C₂ (or C2-C3) bond. The two extreme conformations are the eclipsed conformation (1) and the staggered conformation (II). The staggered conformation is more stable than the eclipsed conformation by 3.4 kcal-mol-1. ### Conformations of n-butane (CH3-CH2-CH2-CH3): n-butane contains two kinds of C-C bonds. So, conformations likely to be generated depends on that particular C-C bond around which C-atoms are made to rotate. i] **Rotation about the C₁-C₂ bond**: Keeping C₁ fixed, when C₂ is rotated around the C₁ - C₂ bond axis, infinite numbers of conformations are obtained. Among these, two principal conformations are eclipsed (1) and staggered (II) conformations. Their order of stability is: staggered > eclipsed, i.e., molecules of n-butane spend most of their time in staggered conformation (II). ii] **Rotation about the C₂-C₃ bond**: Infinite number conformations are possible, if C3 is made to rotate around C₂-C₃ bond axis, keeping C₂ fixed. Among these 4 chief conformations are-anti-staggered (1), gauche staggered (III), eclipsed (II) and fully eclipsed (IV). In anti-staggered conformation, the two -CH₃ groups are anti to each other, ie, they are oriented at an angle of 180° (Φ = 180°). In the gauche staggered conformation, the two-CH3 groups make an angle of 60° with each other (Φ = 60°). In the eclipsed conformation, the two pairs of -CH3 and H and one pair of H-atoms are in direct opposition, while in the fully eclipsed conformation, the two pairs of H-atoms and one pair of CH3 groups are in direct opposition. The order of their stability is: I>>II>IV, ie., the molecules of n-butane pass most of their time in anti-staggered conformation (1). Their Newman projection formulae are shown below: ### Dihedral angle: Dihedral angle (Φ) is the angle between the X-C-C and the C-C-Y plane of X-C-C-Y unit in a molecule. In ethane, it is the angle between the H-Ĉ-C plane and Ĉ-C-H plane. i.e., it is the angle between the C-H bond and the C-H bond in the Newman projection formula. It is also called angle of torsion. ### Warm up Exercise 1. Define conformation and conformational energy. 2. Predict the number of conformations of ethane molecule. 3. Draw the eclipsed and staggered conformations of ethane using Newman projection formula. 4. Define dihedral angle taking ethane as an example. 5. Which one of the eclipsed and staggered conformations of ethane is more stable and why? 6. It is not possible to separate the two extreme conformations of ethane-explain. 7. How can an eclipsed conformation of ethane be converted into a staggered conformation? 8. Write down the names and structures of various conformations of n-butane. 9. In which of the conformations does a molecule of n-butane spend most of its time and why? 10. What is the least stable conformation of n-butane? Write their names and structures and give reason behind such stability. 11. The population of which conformation increases with the rise in temperature? 12. Give examples of a chiral conformation and an achiral conformation of n-butane. 13. Arrange the following conformations of n-butane according to their increasing stability: gauche-staggered (II), fully eclipsed (II), eclipsed (III) and anti-staggered (IV). ## General Methods of Preparation of Alkanes ### 13.5.1 From compounds containing same number of C-atoms ### * **By hydrogenation of unsaturated hydrocarbons (alkenes or alkynes)**: Alkanes may be prepared by reduction of alkenes or alkynes by hydrogen in presence of finely powdered nickel or platinum or palladium catalyst. This process is called catalytic hydrogenation. The pressure and temperature of the reaction depends on the nature of the catalyst used. When a mixture of the vapours of any unsaturated hydrocarbon and hydrogen is passed over nickel catalyst heated at 200 - 300°C, alkanes containing the same number of carbon atoms are obtained. This process is known as Sabatier-Senderens reduction. * **By reduction of alkyl halides**: Alkanes can be prepared by the reduction of alkyl halides with zinc/hydrochloric acid, zinc/acetic acid, zinc/sodium hydroxide, zinc-copper couple/ethanol, aluminium amalgam/ethanol etc. * **By reduction of alkyl halides with sodium borohydride (NaBH4) or hydrogen in the presence of palladium (pd) catalyst**: * **By Clemmensen reduction of aldehydes and ketones**: When aldehydes and ketones are reduced with amalgamated zinc and concentrated hydrochloric acid, the corresponding alkanes are obtained. The reaction is so called after the name of the discoverer. * **By reduction of alcohol, alkyl iodide, aldehyde, ketone and carboxylic acid by red P and HI**: When alcohol, alkyl iodide, aldehyde, ketone and carboxylic acid are reduced by heating with concentrated aqueous solution of hydroiodic acid at 150°C in the presence of a small amount of red phosphorus, the corresponding alkanes are obtained. The reaction is conducted in closed vessel. * **By hydrolysis of Grignard reagents**: When dry and pure metallic magnesium is dissolved in a dry ethereal solution of an alkyl halide, an alkylmagnesium halide (R-MgX) is obtained. This organometallic compound is known as Grignard reagent. In this compound, the carbon atom is directly attached with the Mg-atom and the C-Mg bond is a highly polar covalent bond. When Grignard reagents are treated with water or dilute acids, the corresponding alkanes are obtained in this reaction. The alkyl group (R) of the Grignard reagent takes up a proton to generate alkane (RH). * **By decarboxylation of carboxylic acids**: When a mixture of anhydrous sodium or potassium salt of a carboxylic acid and sodalime (NaOH + CaO) is strongly heated, a molecule of carbon dioxide is eliminated from the acid (decarboxylation) to produce an alkane. * **By Wurtz reaction**: When a dry ethereal solution of an alkyl halide (preferably bromide or iodide) is treated with metallic sodium, the two alkyl groups of two alkyl halide molecules combine to form an alkane. This reaction for the preparation of an alkane is known as Wurtz reaction. The resulting alkane contains twice of the number of carbon atoms present in the molecule of alkyl halide. * **By Kolbe's electrolysis method**: When a cold and concentrated aqueous solution of sodium or potassium salt of a carboxylic acid is electrolysed between platinum electrodes, hydrogen gas and NaOH or KOH are formed at the cathode and at the anode, alkane and CO₂ are obtained. When the mixture of CO2 and alkane is allowed to pass through caustic soda solution, CO2 is absorbed, and the alkane is obtained. This process for the preparation of alkanes is known as Kolbe's electrolysis. * **Corey-House synthesis**: An alkyl halide, RX is first treated with lithium metal in dry ether medium to form alkyl lithium (R-Li) which is then treated with cuprous iodide to form lithium dialkylcuprate (R₂CuLi). Lithium dialkylcuprate is finally treated with a suitable alkyl halide (R'X or RX) to form desired alkane (R-R' or R-R). * **From inorganic carbides**: Some inorganic carbides react with water to liberate saturated hydrocarbons. For example, when beryllium carbide and aluminium carbide are heated with water, they get hydrolysed to form methane. This method gives pure methane. * **From alkyl boranes**: Alkanes may be prepared by treating trialkylboranes, obtained by hydroboration of alkenes with propanoic acid (protonolysis). The document also introduces some important concepts about the conformations of alkanes. For example: - The importance of the conformers such as eclipsed and staggered - The role of dihedral angle and the potential energy of the conformations - The role of the stability of conformations in determining the properties of alkanes The document also provides a detailed description of different methods used to prepare alkanes. It also touches upon potential limitations of each method. This document serves as a good introduction to the world of alkanes. It discusses the unique properties and behaviors of alkanes, as well as their different conformations. The document also highlights the important applications of alkanes in the field of chemistry.