CHAPTER 5 UNSATURATED HYDROCARBONS (ALKENES AND ALKYNES) BY MAHWASH HAFEEZ

ALKENES Alkenes are hydrocarbons that contain a carbon – carbon double bond (with no functional groups, only substituents present). They are also called “Olefins”. They are characterized by molecular formula CnH2n. They are said to be unsaturated (having double bond). They are also called aliphatic hydrocarbons ( chain compounds)

SOME IMPORTANT TERMS FOR NAMING ALKENES For naming the alkenes sytematically first we have to study about the following four features: (a) Root word (b)Primary suffix (c)Secondary suffix (d)Prefix

ROOT WORD:(PARENT CHAIN/MAIN CHAIN) It means the carbon chain ROOT WORD:(PARENT CHAIN/MAIN CHAIN) It means the carbon chain. Some commonly used root words in alkenes are: Chain length Root Word C1 Meth- C2 Eth- C3 Prop- C4 But- C5 Pent- C6 Hex- C7 Hept C8 Oct- C9 Non- C10 Dec-

Primary Suffix: The primary suffixes are added to the root word to show unsaturation(double bond) in a carbon chain. Name of Carbon Chain Primary Suffix IUPAC name Unsaturated -ene Alkene (CH2 ═ CH2)

PREFIX Prefixes are added to the root word to indicate the presence of one or more substituents on the main carbon chain. These prefixes include: fluoro(-F) Chloro(-Cl) bromo(-Br) Iodo(-I) Nitro(-NO2) Alkoxy(-O-R) Alkyl groups

Arrangement of Prefixes, Root word and Suffixes The prefixes, root words, primary and secondary suffixes are arranged as follows while writing the name of an alkene. IUPAC name = Prefixes + Root word + Primary Suffix + Secondary suffix

RULES FOR NAMING ALKENES The base or parent chain of alkene is determined by the longest chain of carbon atoms containing the double bond in the formula. The longest chain may be bend or twist. It is seldom horizontal. CH3 CH3-CH2-CH2-CH2-CH | ║ CH3-CH2-CH2 CH CH | ║ | CH3-CH2-CH-CH2-C-CH3 7 CARBON CHAIN 8 CARBON CHAIN

Numbers are used to specify the locations of the double bonds. If the chain contain only double bond and no other substituents, the chain is numbered starting from the Carbon closest to the double bond and the double bond is given the lower number of its two double-bonded carbon atoms. 1- Butene OR But – 1 – ene A compound with two double bonds is a diene. A triene has three double bonds and a tetraene has four bonds. Numbers are used to specify the locations of the double bonds. 1,3-butadiene buta-1,3-diene

The name of this compound is 3- methy-l- octene or If one substituent or functional group is attached to the parent chain, then numbering should start from the side to give minimum number to the sum of substituent and the double bond. e.g. 1 CH2 ║ 8CH3-7CH2-6CH2 2 CH | | 5CH2-4CH2-3CH-CH3 The name of this compound is 3- methy-l- octene or 3- methyl-oct-1-ene. We will start the numbering from right side because from right side the sum of the numbers of carbon containing substituent and the double bond is 1+3=4 but from left side it is 6+8=14.

EXAMPLES Name the following alkene according to IUPAC system CH2 ═ CH – CH2 – CH2 – CH3 | | CH3 CH3 2,3-dimethyl-1-pentene 2,3-dimethyl-pent-1-ene

1CH2 ═ 2CH – 3CH2 ═ 4CH – 5CH2 – 6CH2 – 7CH3 | | CH3 CH3 2,5- dimethyl – 1,3 – heptadiene 2,5- dimethyl – hepta – 1,3- diene

1CH2 ═ 2CH – 3CH2 ═ 4CH – 5CH2 – 6CH ═ 7CH – 8CH3 | CH3 2 – methyl - 1,3,6-octatriene 2 – methyl - octa-1,3,6-triene

3-ethyl-hepta-1-ene – 5 - ol 2CH ═ 1CH2 | CH3 – CH2 – 3CH – 4CH2 – 5CH2 – 6CH2 – 7CH3 OH 3-ethyl-1-heptene – 5 - ol 3-ethyl-hepta-1-ene – 5 - ol

6-chloro-4-ethyl-6-methyl-1,4-heptadiene Cl | 1CH2 ═ 2CH – 3CH2 – 4C ═ 5CH – 6C – 7CH3 | | CH2 CH3 CH3 6-chloro-4-ethyl-6-methyl-1,4-heptadiene 6-chloro-4-ethyl-6-methylhepta-1,4-diene

Dehydrohelogenation of alkyl halides: SYNTHESIS OF ALKENES Dehydrohelogenation of alkyl halides: Alkenes can be prepared by the removal of a hydrogen and a halogen (i.e. hydrogen halide) from an alkyl halide by heating. CH3 – CH2 – Cl CH2 ═ CH2+ HCl 2. Dehydration of alcohols: Alkenes can be prepared by the removal of a water molecule from an alcohol. CH3 – CH2 – CH2 – OH CH3 – CH ═ CH2+ H2O 3. Hydrogenation of alkynes: Alkenes can be prepared by hydrogenation of alkynes in the presence of a catalyst. CH ≡ CH Pd(BaSO4) CH2 ═ CH2 Quinoline

4. Dehalogenation of vic – dihalides: Alkenes can be prepared by the removal of a halogen from vic-dihalide(compounds in which halogens are attached to adjacent carbon atoms) by treating it with Zn dust in an anhydrous (containing no water) solvent like methanol or acetic acid. CH2 – CH2 + Zn (dust) CH3OH CH2═ CH2 + ZnBr2 | | CH3CO2H Br Br

PHYSICAL PROPERTIES OF ALKENES Physical State: Alkenes with 1-4 Carbon atoms are gases. Alkenes with 5-17 Carbon atoms are volatile liquids. Alkenes with more than 18 Carbon atoms are solids. Melting & Boiling Point: Alkenes usually have low melting and boiling points. Solubility: Alkenes being weekly polar, are soluble in non- polar solvents such as CCl4, ether and benzene. They are generally insoluble in water.

Density: The density of liquid alkenes increases slightly with increase in the size of the molecules due to the increasing intermolecular van der Waals’ forces which cause the alkenes to be more compact in the condensed liquid state. Reactivity: Alkenes are relatively reactive as compared to alkanes.

REACTIONS OF ALKENES Hydrogenation: A molecule of hydrogen can be added to an alkene under moderate pressure (1-5atm) in the presence of a catalyst to produce an alkane. CH2═CH2 + H2 a catalyst CH3 – CH3 1 – 5 atm 2. Halogenation: Alkenes can readily react with halogens in an inert solvent at room temperature to give vic. Dihalide. CH2═CH2 + Br2 CCl4 CH2Br – CH2Br

3. Hydartion: Alkenes react with water to form alcohols. CH3 – CH═CH2+ H – OH H2SO4 CH3 – CH – CH3 | OH 4. Hydrohalogenation: Alkenes react with hydrogen halides to form haloalkanes. CH2═CH2 + HX  CH3 – CH2 – X Mechanism: H H H H H H C=C H–C+–C–H H– C–C–H H H H Br H + H – Br Br-:

5. Oxidation: It can be done in two ways: Epoxidation: Alkenes react with peroxyacids to form cyclic ethers called epoxides. O O O ║ ║ CH2═CH2+CH3–C–O–OHCH2 CH2 + CH3–C–OH Peracetic acid Ethylene oxide Ozonolysis: Alkenes react with ozone (O3) to form ozonides. O – O CH2 ═CH2 + O3  CH2 CH2 O An Ozonide

6. Polymerization: Alkenes can be polymerized at high temperature to give alkanes. n(CH2═CH2)  –( CH2 – CH2)n– Formation of polymers : heat CH2 = CH2 [CH2 - CH2]n ethylene pressure polyethylene CH3 heat CH3 CH = CH2 CH2 - CH2 n propene pressure polypropylene Cl heat Cl CH = CH2 CH2 - CH2 n vinyl chloride pressure polyvinyl chloride

USES OF POLYMERS Medical uses of polymer 1) Heart valves 2) Blood vessel 3) Surgical mesh 4) Disposable syringes 5) Drug containers 6) Polyethylene is used in formation of food bags 7) Polypropylene is used in formation of water containers and breast implants 8) Polyvinyl chloride is used as coating material (toxic)

TYPES OF POLYMERS 1) Natural polymer : starch, cellulose, rubber, protein 2) Synthetic polymer: plastic, nylon, rayon, Dacron and poly ethylene This pattern repeats itself over and over again Polypropylene