Alkanes & Cycloalkanes PDF

ALKANES &CYCLOALKANES Dr. Omnia Hussein Dr.Omniat Nageeb.m.a  Alkanes I. introduction of alkane II. Structure of alkane III. Nomenclature of alkane IV. Physical properties of alkane V. Isomerism of alkane VI. Method of preparation VI...

ALKANES &CYCLOALKANES Dr. Omnia Hussein Dr.Omniat Nageeb.m.a  Alkanes I. introduction of alkane II. Structure of alkane III. Nomenclature of alkane IV. Physical properties of alkane V. Isomerism of alkane VI. Method of preparation VII.Chemical reaction I-Introduction of Alkanes 3  II-Structures of Alkanes The alkanes are hydrocarbons that only contain single covalent bonds between their carbon atoms. This means that they are: 1. saturated compounds 2. has no specific reactive functional groups 3. and are quite unreactive.  These are saturated hydrocarbons because they have the maximum number of hydrogen atoms. Alkanes are also called Paraffins because of little reactivity towards reagents. Members of a homologous series with general formula CnH2n+2  III-Nomenclature of Alkanes There are two systems of naming these hydrocarbons: (1) The Common System: (Alkane and alkyl group) The alkanes are naming by prefixing (meth... eth... prop..., etc.) which indicates the number of carbons in the main, or parent, chain of the molecule, and the suffix...ane to indicate that the molecule is an alkane. The Alkane having a straight or normal chain are called normal (or n-)hydrocarbons. This indicated by prefixing n- to the name of alkane. The Alkane having branches are called branched chain hydrocarbons. Such alkanes as possess a one- carbon branch on 2nd carbon of chain are referred to as Iso -hydrocarbon  From pentane (C5) 0n ward, when the normal C chain present in the molecules has 2 0ne carbon branches on 2nd C from the end the Alkane referred to as Neo hydrocarbon To assign the prefixes sec-, which stands for secondary, and tert-, for tertiary, it is important that we first learn how to classify carbon molecules. If a carbon is attached to only one other carbon, it is called a primary carbon. If a carbon is attached to two other carbons, it is called a secondary carbon. A tertiary carbon is attached to three other carbons and last, a quaternary carbon is attached to four other carbons. Examples:  Alkyl Groups: An alkyl group is formed by removing one hydrogen from the alkane chain and is described by the formula CnH2n+1. The removal of this hydrogen results in a stem change from -ane to -yl. Take a look at the following examples: 2)The IUPAC rules for naming linear and branched alkanes are given below: A] The selection of parent chain: 1.Choose the longest, most substituted carbon chain containing a functional group. B] Numbering the parent chain: 1- A carbon bonded to a functional group must have the lowest possible carbon number. If there are no functional groups, then any substituent present must have the lowest possible number. 2- Take the alphabetical order into consideration; that is, after applying the first two rules given above, make sure that your substituents and/or functional groups are written in alphabetical order. Notes: o The groups: sec-butyl and tert-butyl are alphabetized under "b". However the Isobutyl and Isopropyl groups are alphabetized under "i" and not under "b" or "p". o In the following molecule, 4-ethyl-5-methyloctane, both methyl and ethyl groups are at equivalent positions. However the ethyl group comes first in the alphabetical order. Therefore it is to be written first in the name and to be given the lowest number. o In the following molecule, 5-ethyl-2-methylheptane, the methyl and ethyl groups are not at equivalent positions. The methyl group is given the least number according to the rule of first point of difference. C] Grammar to be followed in writing the IUPAC name: i) The IUPAC name must be written as one word. However, there are exceptions. ii) The numbers are separated by commas. iii) The numbers and letters are separated by hyphens. iv) If there are two or more same type of simple substituents they should be prefixed by di, tri, tetra, penta etc. If the side chains themselves contain terms like di, tri, tetra etc., the multiplying prefixes like bis, tris, tetrakis etc., should be used. E.g. The two 1,2-dimethylpropyl groups are indicated by the prefix "bis" as shown below.  IV- Physical Properties of Alkane : 1. Alkanes have no color 2. Alkanes are lighter than water and have a lower density 3. Alkanes dissolve more readily in non-polar than polar solvents because they are nonpolar molecules 4. Alkanes do not dissolve in water 5. The melting and boiling temperatures of shorter chain alkanes are low, but as the number of carbon atoms in the carbon chain rises, the melting and boiling values of alkanes rise. 6. The first four alkanes is gases, the next thirteen alkanes are liquids. The higher member are waxy solid  V-ISOMERISM OF ALKANES : ❖ Different compounds that share the same molecular formula are known as isomers. Alkanes exhibit structural isomerism. ❖The First three hydrocarbon( H.C.)of series(methane, Ethane & propane) do not exhibit isomerism. Butane exhibits isomerism since the straight- chain structure and the branched chain structure represent the two isomers of butane, C4H10. ❖All higher H.C. show chain isomerism and the number of isomers goes on increasing rapidly with the increase in the number of C atoms 19 ❖No. of Isomers: butane has 2 , pentane has 3, hexane has 5….etc 20 VI-METHODS OF PREPARATION 1. From Alkenes & Alkynes 2. From Alkyl Halid 3. From Carboxylic Acid 4. From Alcohol, Aldehyde, ketone & Carboxylic acids 21 1. From Alkenes & Alkynes Alkanes are formed by passing a mixture of unsaturated H C & hydrogen over finely divided nickel(or Pt, Pd) at 200- 300o This method is used for industrial preparation of Alkanes. 22 2. From Alkyl Halide The alkanes can prepare from Alkyl halides by: 1- Action of Sodium on Alkyl Halides (Wurtz reaction) A solution of alkyl halide in ether on heating with sodium gives alkane. R-X + 2Na + X-R R-R + 2NaX An alkyl halide on Wurtz reaction leads to the formation of symmetrical alkane having an even number of carbon atoms. Two different alkyl halides, on Wurtz reaction give all possible alkanes. CH3X + Na + C2H5X → CH3CH2CH3 + CH3CH3 + CH3CH2CH2CH+2NaX 23 Mechanisms of Wurtz reaction First Mechanism: By a formation of free radicals as an intermediate. ❖This mechanism works when the reaction will be performed in the vapour phase. ❖ The Vapour phase is considered a suitable phase for free radicals. ❖The steps taken in the reaction are as follow: 24 Step 1: A transfer of one electron from a sodium atom makes a free radical of alkyl R–X + Na → R + Na+X- Step 2: In the second step, the second sodium atom releases one more electron to the free radical and provides a carbonium ion. R + Na → R−Na+ Step 3: A halide ion is displaced by an alkyl anion from another molecule of alkyl halide. This reaction is considered an SN2 reaction. R−Na+ + R–X → R–R + Na+X- 25 Second Mechanism: This Ionic mechanism uses an organometallic compound as an intermediate and the reaction is performed in a solution. CH3-CH2-I + 2Na →CH3-CH2-Na(+) + NaI CH3-CH2-Na(+) + CH3-CH2-I →C2H5- C2H5 +NaI 26 2- Reduction of Alkyl Halides 27 3- From Carboxylic Acid The alkanes can prepare from carboxylic acid by: 1- Decarboxylation of carboxylic acid When the Na salt of carboxylic acid is heated strongly with sodalime (NaOH +CaO), a molecule of CO2 is split off as carbonate and alkane is formed( has one carbon atom less than original 28 2- Electrolysis of salt of carboxylic acid( Kolbe’s method) The Kolbe reaction is a radical reaction. An aqueous solution of sodium or potassium salt of carboxylic acid is electrolyzed in this reaction, resulting in the dissociation of the salt into carboxylate ion and sodium or potassium ions. For the production of ethane and higher alkanes, this process is used. 29  VII-CHEMICAL REACTION : Alkanes are relatively stable to most of the common reagent at room temperature. The relative stability or inactivity of Alkanes may be explained considering the nature of C-C & C-H bond present in Alkanes. Where this bonds are short & strong. Alkanes gives only two types of reaction: 1. Substitution reaction. 2.Thermal & Catalytic reaction. 30 1- Substitution reaction. In these reactions, one or more of H- atoms of alkane are substitutes by either Atoms like Halogens or group like NO2 , SO3H. A] Halogenation: Alkanes undergo a substitution reaction with halogens in the presence of light. For instance, in ultraviolet light, methane reacts with halogen molecules such as chlorine and bromine. The order of reactivity of halogen is F > Cl > Br > I 31 Mechanisms of Halogenation of alkane: The mechanism is free radical substitution reaction. 1. Initiation Step: The Cl-Cl bond of elemental chlorine undergoes hemolysis when irradiated with UV light, and this process yields two chlorine atoms, also called chlorine radical 32 2. Propagation Step: A chlorine radical abstracts a hydrogen atom from methane to produce the methyl radical. The methyl radical in turn abstracts a chlorine atom from a chlorine molecule and chloromethane is formed. The second step of propagation also regenerates a chlorine atom. These steps repeated many times until termination occurs 33 3. Termination Step: Termination takes place when a chlorine atom reacts with another chlorine atom to generate Cl2, or chlorine atom can react with a methyl radical to form chloromethane which constitutes a minor pathway by which the product is made. Two methyl radicals can also combine to produce ethane, a very minor by product of this reaction. The reaction does not stop at this step, however because the chlorinated methane product can react with additional chlorine to produce polychlorinated products. 34 35 B] Nitration & sulphenation: Another reaction of commercial importance is the nitration & sulphenation of alkanes Such reactions usually are carried out in the vapor phase at elevated temperatures using nitric acid ( HNO3 ) or nitrogen tetroxide ( N2O4 ) as the nitrating agent & sulphonic acid group(SO3H) as sulphenating agent All available evidence points to a radical mechanism for nitration & sulphenation. Mixtures are obtained; nitration of propane gives not only 1- and 2- nitropropanes but nitroethane and nitromethane: 36 37 2-Thermal & Catalytic reaction. A-Pyrolysis( Cracking ): Pyrolysis is defined as the conversion of a compound into smaller fragments in the absence of air through the application of heat”. It is different from combustion. It happens in the absence of air and hence oxidation of compounds does not take place. Generally, pyrolysis of alkanes is also named as cracking. The cracking of alkanes follows a free radical mechanism 38 B] Isomerisation: Isomerisation of alkanes involves the conversion of straight-chain alkanes into isomeric branched chain alkanes. For example, when hexane is heated with aluminium chloride in the presence of dry HCl gas at 573K under about 35 atmospheres pressure, 2and 3-methyl pentane is produced. 39 C] Aromatisation: The process of conversion of aliphatic compound into aromatic compound is known as aromatization. Alkanes having six to 10 carbon atoms are converted into benzene and its homologues at high pressure and temperature in presence of catalyst. 40 CYCLOALKANES 41  I-STRUCTURE OF CYCLOALKANES Cycloalkanes are alkanes which have some of their carbon atoms arranged in a ring. Rings of different sizes beginning with three carbons are possible. Cycloalkanes are saturated since all the carbon atoms that make up the ring are single bonded to other atoms. Because of the ring, a cycloalkane has two fewer hydrogens than an acyclic (noncyclic) alkane with the same number of carbons. The general molecular formula for cycloalkanes is CnH2n 42 43  II-NOMENCLATURE OF CYCLOALKANES Cycloalkanes are commonly drawn as line structures whereby each vertex represents a carbon understood to be connected to an appropriate number of hydrogens to give carbon four bonds. Cycloalkanes are named by adding the prefix “cyclo” to the ‘alkane’ name that has the same number of carbon atoms as those in the ring. These most common cycloalkanes are represented as shown below 44 Naming Substituted Cycloalkanes The rules for naming cycloalkanes are similar to those used for straight- chain alkanes: i. The parent ring is the largest ring in the molecule. ii. The parent name is generated by adding the prefix cyclo- to the name of the alkane with the same number of carbons. iii. Identify the substituents and their location by numbering the ring from the carbon containing substituents so as to give the substituents the lowest possible location numbers. iv. Alphabetize the substituents in the full name of the cycloalkane. 45 46  III-CYCLOALKANES- PROPERTIES Cycloalkanes are types of alkanes that have one or more rings of carbon atoms in their structure. The physical properties of cycloalkanes are similar to those of alkanes, but they have higher boiling points, melting points and higher densities 47  V-METHODS OF PREPARATION 1. From Di-halogen Compounds: - Suitable 1,3 or 1,4 like di- halogen alkanes on treatment with sodium or zinc give corresponding cycloalkanes. 2. From Aromatic Compounds. Benzene may be catalytically hydrogenated at elevated temperature and pressure to yield cyclohexane. 48  VI-CHEMICAL REACTION: 1. Hydrogenation: (ring opening) Cycloalkanes undergo hydrogenation in the presence of catalysts like Ni or Pt to form the corresponding saturated hydrocarbons. The ease of hydrogenation decreases as the size of the ring increases. Higher Cycloalkanes having six or more carbon atoms are usually stable to hydrogenation. 49 50 2. Halogenation: a. Addition reaction of halogen: (Leading to ring opening) Cyclopropane reacts with chlorine and bromine in dark to form addition products. CCl4 is used as the solvent Cyclobutane and higher members do not give this reaction. 51 b. Substitution reaction with halogen: Cycloalkanes react with chlorine and bromine in the presence of UV light to give substitution products. 52