Chapter 16: Hydrocarbons PDF

# Chapter 16: Hydrocarbons ## The bonds in cyclopropane molecule The bonds in cyclopropane molecule are directed in three-dimension so it is not planar. The C-C-C angle is 60° whereas H-C-H and C-C-H angles are 109.5°. The 3D shape of cyclopropane molecule is shown. - H - H - C - 109.5° - 60°...

# Chapter 16: Hydrocarbons ## The bonds in cyclopropane molecule The bonds in cyclopropane molecule are directed in three-dimension so it is not planar. The C-C-C angle is 60° whereas H-C-H and C-C-H angles are 109.5°. The 3D shape of cyclopropane molecule is shown. - H - H - C - 109.5° - 60° - C - C - H - H - H - H - 109.5° **Shape of cyclopropane** ## Nomenclature of Alkenes See section 15.4.3 ## 16.4 PREPARATION OF ALKENES Alkenes have several methods of preparation. ## Free radical substitution reaction Free radical substitution reaction requires ultraviolet light for free radical formation, whereas cracking requires heating to break large molecules and convert them to smaller, more useful molecules. We will study free radical substitution reaction of ethane with its mechanism. ### 16.2.1 Free Radical Substitution of Ethane Substitution reactions of ethane involve halogen free radicals, which are very reactive species and attack on ethane molecules. There are three steps in this reaction. 1. **Initiation** The ultraviolet light breaks the chlorine molecule via homolytic fission and gives chlorine free radicals. ``` Cl-Cl -> Cl + Cl ``` 2. **Propagation** This step is named propagation because the substitution of ethane progresses like a chain reaction. As soon as the chlorine radical is formed in the initiation step, it attacks on ethane molecule to give ethyl free radical. ``` H-C-CH + Cl -> H-C-CH + Cl ``` Ethyl free radical reacts with chlorine molecule to form chloroethane and another chlorine free radical. ``` H-C-CH + Cl -> H-C-CH-Cl + Cl ``` If we use excessive chlorine, the propagation reaction continues giving chlorine free radicals and chloro substituted alkyl free radicals, each step gets one free radical at a time. Resultantly, we get a mixture of chlorosubstituted alkanes (like 1,1-dichloroethane, 1,1,1-trichloroethane, 1,1,2,2-tetrachloroethane, etc.). The final product, in the presence of enough chlorine, is 1,1,1,2,2,2-hexachloroethane (C2Cl6). 3. **Termination** This is the end of free radicals and can take place at any stage in propagation step. In the termination stage, there is a possibility of reaction among different free radicals formed in the propagation step. ## 16.3 SHAPES OF ETHANE AND CYCLOPROPANE ### Shape of Ethane Molecules (C2H6) Ethane molecule has two carbon atoms bonded by single covalent bonds with one another. To each carbon atom, three hydrogen atoms are bonded by single covalent bond. A covalent bond, which is formed by end-on overlap of atomic orbitals, is called a sigma (σ) bond. Remember that each single covalent bond is a sigma (σ) bond. When it comes to hybridization of atoms, each carbon atom in ethane molecule undergoes sp³ hybridization. Each orbital shared by the two carbon atoms, either with hydrogen atoms or with one another, is sp³ hybridized orbital. Ethane molecule is not planar because all covalent bonds around carbon atoms make an angle of 109.5° with one another. This arrangement of atoms in the space gives tetrahedral shape to the atoms around each carbon atom. ### Shape of Cyclopropane Molecules (C3H6) The molecule of cyclopropane has three carbon atoms bonded by single covalent bonds with one another in the form of a ring. There are two hydrogen atoms bonded by single covalent bond with each carbon atom. In cyclopropane molecule, each carbon atom shows sp³ hybridization. The orbitals shared between carbon atoms or with hydrogen atoms are sp³ hybridized orbitals with tetrahedral arrangement. ## Unsaturated Hydrocarbons Unsaturated hydrocarbons have at least one multiple (double or triple) bond. Since doubly bonded carbon is bonded to three atoms while triply bonded carbon to two atoms, so more atoms can be added to alkenes and alkynes and are called unsaturated hydrocarbons. They include alkenes and alkynes. ### a. Alkenes Alkenes are commonly known as olefins. They contain at least one double bond in their parent chain. Each carbon having one double bond is sp² hybridized. If they have more than one double bond, they are called polyenes. ### b. Alkynes The common name of alkynes are acetylenes. They contain at least one triple bond between carbon atoms. Each triply bonded carbon is sp hybridized. Those alkynes which have more than one triple covalent bonds are called polyynes. ## 16.1.2 Closed Chain Hydrocarbons Closed chain hydrocarbons have no terminal carbon in the principal carbon skeleton. All carbon atoms are bonded to, at least, two other carbon atoms. They are divided into two classes. ### 1. Alicyclic Hydrocarbons The word alicyclic is a blend of two words, aliphatic and cyclic, which means that they are aliphatic hydrocarbons with rings of carbon atoms. They show similarity in characteristics to aliphatic hydrocarbons. Alicyclic hydrocarbons have two hydrogen atoms lesser than their relevant open chain hydrocarbons. ## Aromatic Hydrocarbons Aromatic hydrocarbons have at least one benzene ring in their molecules. The simplest aromatic hydrocarbon is benzene. Benzene molecule is hexagonal in shape with alternating double covalent bonds. Each carbon of benzene ring is sp² hybridized. ## Concept Assessment Exercise 16.1 1. Differentiate between saturated and unsaturated compounds. Give examples. 2. Define aliphatic and aromatic compounds. Give two examples of each. 3. What type of hybridization is shown by each carbon of hexa-1,2-diene? ## 16.2 REACTIVITY OF ALKANES Alkanes are non-polar hydrocarbons because the electronegativity difference between carbon and hydrogen is not enough to declare them polar. Alkanes are also called paraffins. The word paraffins is a combination of two Latin words "parum" means little and "affinis" means reactivity. The wordly meaning of alkanes is that they are less reactive. The low reactivity of alkanes is attributed to two factors: 1. **Non-polar Bonds** The electronegativity values of carbon and hydrogen atoms are 2.5 and 2.1 respectively. The difference in their electronegativity is negligible so bonding electrons are symmetrically distributed among them and the carbon-hydrogen bonds in alkanes are non-polar. Since alkanes are non-polar so acids, bases, oxidizing agents and reducing agents will not react with alkanes. 2. **Strength of Sigma Bond** Alkanes have only sigma (σ) bonds, they do not have pi (π) bonds. Sigma (σ) bond is a strong bond because its electrons lie very close to the bonding atoms (nuclei) and are strongly held by them. The sigma (σ) electrons in alkanes cannot be easily utilized for chemical reactions which reflects their inertness. Although, alkanes do not react with other reagents under normal conditions but they do react with some substances at high temperature. Combustion of alkanes (burning sui gas in kitchen) is a common reaction in our daily life. The two important reactions shown by alkanes are given: **a. Free Radical Substitution Reaction** **b. Cracking** ## Hydrocarbons Hydrocarbons are organic compounds which have hydrogen and carbon elements only. Hydrocarbons are broadly divided into two classes, i.e. aliphatic hydrocarbons and aromatic hydrocarbons. The word aliphatic and aromatic come from Greek language meaning "fatty" and "fragrant" respectively. Aliphatic hydrocarbons include open chain hydrocarbons or closed chain hydrocarbons that resemble open chain hydrocarbons. On the contrary, aromatic compound include benzene and compounds whose properties resemble benzene. ## Classification of Hydrocarbons ### Open Chain (Acyclic) Hydrocarbons - **Saturated Hydrocarbons (Alkanes) (Paraffins)** - C2H6 - Ethane - **Unsaturated Hydrocarbons** - Alkenes (Olefins) - Alkynes (Acetylenes) - H2C=CH2 - Ethene - CH≡CH - Ethyne ### Closed Chain (Cyclic) Hydrocarbons - **Alicyclic Hydrocarbons (Cycloalkanes)** - cyclopentane - **Aromatic Hydrocarbons (Having benzene ring)** - Benzene ## 16.1.1 Open Chain Hydrocarbons Open chain hydrocarbons have a carbon skeleton in which carbon atoms form chains, either straight or branched. They are also named acyclic hydrocarbons, which means that they have no cycle (ring). There must be terminal carbon atoms in the chain. They are divided into two classes: 1. **Saturated hydrocarbons** Saturated hydrocarbons have all single covalent bonds among their carbon atoms. Their carbon atoms are bonded to four atoms so no more atoms can be added to them and are named saturated hydrocarbons. Each singly bonded carbon is sp³ hybridized. They may be straight chain or branched chain. Alkanes are examples of saturated hydrocarbons: - CH3-CH2-CH2-CH3 (butane (straight chain alkane)) - CH3 - CH3-CH-CH3 (2-methylpropane (branched chain alkane))