Organic Chemistry – Some Basic Principles and Techniques

Unit The development of electronic theory of covalent bonding ushered organic chemistry into its modern shape. . TETRAVALENCE OF CARBON: SHAPES OF ORGANIC COMPOUNDS . .

The Shapes of Carbon Compounds The knowledge of fundamental concepts of molecular structure helps in understanding and predicting the properties of organic compounds. You have already learnt theories of valency and molecular structure in Unit . Also, you already know that tetravalence of carbon and the formation of covalent bonds by it are explained in terms of its electronic configuration and the hybridisation of s and p orbitals. It may be recalled that formation and the shapes of molecules like methane (CH ), ethene (C H ), ethyne (C H ) are explained in terms of the use of sp , sp and sp hybrid orbitals by carbon atoms in the respective molecules.

Hybridisation influences the bond length and bond enthalpy (strength) in compounds. The sp hybrid orbital contains more s character and hence it is closer to its nucleus and forms shorter and stronger bonds than the sp hybrid orbital. The sp hybrid orbital is intermediate in s character between sp and sp and, hence, the length and enthalpy of the bonds it forms, are also intermediate between them. The change in hybridisation affects the electronegativity of carbon.

The greater the s character of the hybrid orbitals, the greater is the electronegativity. Thus, a carbon atom having an sp hybrid orbital with % s character is more electronegative than that possessing sp or sp hybridised orbitals. This relative electronegativity is reflected in several physical and chemical properties of the molecules concerned, about which you will learn in later units. .

. Some Characteristic Features of π Bonds In a π (pi) bond formation, parallel orientation of the two p orbitals on adjacent atoms is necessary for a proper sideways overlap. Thus, in H C=CH molecule all the atoms must be in the same plane. The p orbitals are mutually parallel and both the p orbitals are perpendicular to the plane of the molecule.

Rotation of one CH fragment with respect to other interferes with maximum overlap of p orbitals and, therefore, such rotation about carbon-carbon double bond (C=C) is restricted. The electron charge cloud of the π bond is located above and below the plane of bonding atoms. This results in the electrons being easily available to the attacking reagents. In general, π bonds provide the most reactive centres in the molecules containing multiple bonds.

Problem . How many σ and π bonds are present in each of the following molecules? (a) HC ≡ CCH=CHCH (b) CH =C=CHCH (a) σ C – C : ; σ C–H : ; π C=C : ; π C ≡ C: (b) σ C – C : ; σ C–H : ; π C=C : . Problem .

What is the type of hybridisation of each carbon in the following compounds? (a) CH Cl, (b) (CH ) CO, (c) CH CN, (d) HCONH , (e) CH CH=CHCN (a) sp , (b) sp , sp , (c) sp , sp , (d) sp , (e) sp , sp , sp , sp Problem . Write the state of hybridisation of carbon in the following compounds and shapes of each of the molecules. (a) H C=O, (b) CH F, (c) HC ≡ N.

(a) sp hybridised carbon, trigonal planar; (b) sp hybridised carbon, tetrahedral; (c) sp hybridised carbon, linear. . STRUCTURAL RepresenTATIONS OF organic COMPOUNDs . .

Complete, Condensed and Bond-line Structural Formulas Structures of organic compounds are represented in several ways. The Lewis structure or dot structure, dash structure, condensed structure and bond line structural formulas are some of the specific types. The Lewis structures, however, can be simplified by representing the two-electron covalent bond by a dash (–). Such a structural formula focuses on the electrons involved in bond formation.

A single dash represents a single bond, double dash is used for double bond and a triple dash represents triple bond. Lone- pairs of electrons on heteroatoms ( e.g ., oxygen, nitrogen, sulphur, halogens etc.) may or may not be shown. Thus, ethane (C H ), ethene (C H ), ethyne (C H ) and methanol (CH OH) can be represented by the following structural formulas. Such structural representations are called complete structural formulas .

Similarly, CH CH CH CH CH CH CH CH can be further condensed to CH (CH ) CH . For further simplification, organic chemists use another way of representing the structures, in which only lines are used. In this bond-line structural representation of organic compounds, carbon and hydrogen atoms are not shown and the lines representing carbon-carbon bonds are drawn in a zig-zag fashion. The only atoms specifically written are oxygen, chlorine, nitrogen etc.

The terminals denote methyl (–CH ) groups (unless indicated otherwise by a functional group), while the line junctions denote carbon atoms bonded to appropriate number of hydrogens required to satisfy the valency of the carbon atoms. Some of the examples are represented as follows: (i) -Methyloctane can be represented in various forms as: (a) CH CH CH CH CH CH CH CH | These structural formulas can be further abbreviated by omitting some or all of the dashes representing covalent bonds and by indicating the number of identical groups attached to an atom by a subscript. The resulting expression of the compound is called a condensed structural formula . Thus, ethane, ethene, ethyne and methanol can be written as: CH CH H C=CH HC ≡ CH CH OH Ethane Ethene Ethyne Methanol Ethane Ethene Ethyne Methanol (ii) Various ways of representing -bromo butane are: (a) CH CHBrCH CH (b) (c) (b) (c) In cyclic compounds, the bond-line formulas may be given as follows: Cyclopropane Cyclopentane chlorocyclohexane Problem .

Expand each of the following condensed formulas into their complete structural formulas. (a) CH CH COCH CH (b) CH CH=CH(CH ) CH (a) (b) (b) Condensed formula: (a) HO(CH ) CH(CH )CH(CH ) (b) HOCH(CN) Bond-line formula: Problem . For each of the following compounds, write a condensed formula and also their bond-line formula. (a) HOCH CH CH CH(CH )CH(CH )CH (b) (a) Problem .

Expand each of the following bond-line formulas to show all the atoms including carbon and hydrogen (a) (b) (c) (d) Framework model Ball and stick model Space filling model Fig. . . .

Three-Dimensional Representation of Organic Molecules The three-dimensional ( -D) structure of organic molecules can be represented on paper by using certain conventions. For example, by using solid ( ) and dashed ( ) wedge formula, the -D image of a molecule from a two-dimensional picture can be perceived. In these formulas the solid-wedge is used to indicate a bond projecting out of the plane of paper, towards the observer. The dashed-wedge is used to depict the bond projecting out of the plane of the paper and away from the observer.

Wedges are shown in such a way that the broad end of the wedge is towards the observer. The bonds lying in plane of the paper are depicted by using a normal line (—). -D representation of methane molecule on paper has been shown in Fig. .

Fig. . Wedge-and-dash representation of CH Molecular Models Molecular models are physical devices that are used for a better visualisation and perception of three-dimensional shapes of organic molecules. These are made of wood, plastic or metal and are commercially available.

Commonly three types of molecular models are used: ( ) Framework model, ( ) Ball-and-stick model, and ( ) Space filling model. In the framework model only the bonds connecting the atoms of a molecule and not the atoms themselves are shown. This model emphasizes the pattern of bonds of a molecule while ignoring the size of atoms. In the ball-and-stick model, both the atoms and the bonds are shown.

Balls represent atoms and the stick denotes a bond. Compounds containing C=C (e.g., ethene) can best be represented by using springs in place of sticks. These models are referred to as ball-and-spring model. The space-filling model emphasises the relative size of each atom based on its van der Waals radius.

Bonds are not shown in this model. It conveys the volume occupied by each atom in the molecule. In addition to these models, computer graphics can also be used for molecular modelling. .

Classification of Organic Compounds The existing large number of organic compounds and their ever -increasing numbers has made it necessary to classify them on the basis of their structures. Organic compounds are broadly classified as follows: I. Acyclic or open chain compounds These compounds are also called as aliphatic compounds and consist of straight or branched chain compounds, for example: (homocyclic). Cyclohexane Cyclohexene Cyclopropane Sometimes atoms other than carbon are also present in the ring (heterocylic).

Tetrahydrofuran given below is an example of this type of compound: Tetrahydrofuran These exhibit some of the properties similar to those of aliphatic compounds. (b) Aromatic compounds Aromatic compounds are special types of compounds. You will learn about these compounds in detail in Unit . These include benzene and other related ring compounds (benzenoid).

Like alicyclic compounds, aromatic comounds may also have hetero atom in the ring. Such compounds are called hetrocyclic aromatic compounds. Some of the examples of various types of aromatic compounds are: Benzenoid aromatic compounds Benzene Aniline Naphthalene Non-benzenoid compound Tropone Isobutane Acetaldehyde Acetic acid CH CH Ethane II Cyclic or closed chain or ring compounds (a) Alicyclic compounds Alicyclic (aliphatic cyclic) compounds contain carbon atoms joined in the form of a ring Heterocyclic aromatic compounds Furan Thiophene Pyridine Organic compounds can also be classified on the basis of functional groups, into families or homologous series. .

. Functional Group The functional group is an atom or a group of atoms joined to the carbon chain which is responsible for the characteristic chemical properties of the organic compounds. The examples are hydroxyl group (–OH), aldehyde group (–CHO) and carboxylic acid group (– COOH) etc. .

. Homologous Series A group or a series of organic compounds each containing a characteristic functional group forms a homologous series and the members of the series are called homologues . The members of a homologous series can be represented by general molecular formula and the successive members differ from each other in molecular formula by a –Ch unit. There are a number of homologous series of organic compounds.

Some of these are alkanes, alkenes, alkynes, haloalkanes, alkanols, alkanals, alkanones, alkanoic acids, amines etc. It is also possible that a compound contains two or more identical or different functional groups. This gives rise to polyfunctional compounds. .

NOMENCLATURE OF ORGANIC COMPOUNDS Organic chemistry deals with millions of compounds. In order to clearly identify them, a systematic method of naming has been developed and is known as the IUPAC (International Union of Pure and Applied Chemistry) system of nomenclature. In this systematic nomenclature, the names are correlated with the structure such that the reader or listener can deduce the structure from the name. Before the IUPAC system of nomenclature, however, organic compounds were assigned names based on their origin or certain properties.

For instance, citric acid is named so because it is found in citrus fruits and the acid found in red ant is named formic acid since the Latin word for ant is formica . These names are traditional and are considered as trivial or common names . Some common names are followed even today. For example, Buckminsterfullerene is a common name given to the newly discovered C cluster (a form of carbon) noting its structural similarity to the geodesic domes popularised by the famous architect R.

Buckminster Fuller. Common names are useful and in many cases indispensable, particularly when the alternative systematic names are lengthy and complicated. Common names of some organic compounds are given in Table . .

Table . Common or Trivial Names of Some Organic Compounds . . The IUPAC System of Nomenclature A systematic name of an organic compound is generally derived by identifying the parent hydrocarbon and the functional group(s) attached to it.

See the example given below. By further using prefixes and suffixes , the parent name can be modified to obtain the actual name. Compounds containing carbon and hydrogen only are called hydrocarbons. A hydrocarbon is termed saturated if it contains only carbon-carbon single bonds.

The IUPAC name for a homologous series of such compounds is alkane . Paraffin (Latin: little affinity) was the earlier name given to these compounds. Unsaturated hydrocarbons are those, which contain at least one carbon- carbon double or triple bond. .

. IUPAC Nomenclature of Alkanes Straight chain hydrocarbons : The names of such compounds are based on their chain structure, and end with suffix ‘- ane ’ and carry a prefix indicating the number of carbon atoms present in the chain (except from CH to C H , where the prefixes are derived from trivial names). The IUPAC names of some straight chain saturated hydrocarbons are given in Table . .

The alkanes in Table . differ from each other by merely the number of -CH groups in the chain. They are homologues of alkane series. In order to name such compounds, the names of alkyl groups are prefixed to the name of parent alkane.

An alkyl group is derived from a saturated hydrocarbon by removing a hydrogen atom from carbon. Thus, CH becomes -CH and is called methyl group . An alkyl group is named by substituting ‘ yl ’ for ‘ ane ’ in the corresponding alkane. Some alkyl groups are listed in Table .

. Table . Some Alkyl Groups Table . IUPAC Names of Some Unbranched Saturated Hydrocarbons Branched chain hydrocarbons : In a branched chain compound small chains of carbon atoms are attached at one or more carbon atoms of the parent chain.

The small carbon chains (branches) are called alkyl groups. For example: CH –CH–CH –CH CH –CH–CH –CH–CH CH CH CH (a) (b) Abbreviations are used for some alkyl groups. For example, methyl is abbreviated as Me, ethyl as Et, propyl as Pr and butyl as Bu. The alkyl groups can be branched also.

Thus, propyl and butyl groups can have branched structures as shown below. CH -CH- CH -CH -CH- CH -CH-CH -    Isopropyl- sec-Butyl- Isobutyl- CH -C- CH -C-CH - tert-Butyl- Neopentyl- Common branched groups have specific trivial names. For example, the propyl groups can either be n -propyl group or isopropyl group. The branched butyl groups are called sec -butyl, isobutyl and tert -butyl group.

We also encounter the structural unit, –CH C(CH ) , which is called neopentyl group. Nomenclature of branched chain alkanes: We encounter a number of branched chain alkanes. The rules for naming them are given below. separated from the groups by hyphens and there is no break between methyl and nonane.] .

If two or more identical substituent groups are present then the numbers are separated by commas. The names of identical substituents are not repeated, instead prefixes such as di (for ), tri (for ), tetra (for ), penta (for ), hexa (for ) etc. are used. While writing the name of the substituents in alphabetical order, these prefixes, however, are not considered.

Thus, the following compounds are named as: CH -CH-CH -CH-CH CH  C  CH  CH  CH  , -Dimethylpentane , , -Trimethylpentane H C H C CH  CH  CH  C  CH  CH  CH  -Ethyl- , -dimethylheptane . If the two substituents are found in equivalent positions, the lower number is given to the one coming first in the alphabetical listing . Thus, the following compound is -ethyl- -methyloctane and not -ethyl- -methyloctane. CH — CH —CH—CH —CH —CH—CH —CH CH CH .

The branched alkyl groups can be named by following the above mentioned procedures. However, the carbon atom of the branch that attaches to the root alkane is numbered as exemplified below. CH –CH–CH –CH– CH CH , -Dimethylbutyl- . First of all, the longest carbon chain in the molecule is identified .

In the example (I) given below, the longest chain has nine carbons and it is considered as the parent or root chain. Selection of parent chain as shown in (II) is not correct because it has only eight carbons. . The carbon atoms of the parent chain are numbered to identify the parent alkane and to locate the positions of the carbon atoms at which branching takes place due to the substitution of alkyl group in place of hydrogen atoms.

The numbering is done in such a way that the branched carbon atoms get the lowest possible numbers. Thus, the numbering in the above example should be from left to right (branching at carbon atoms and ) and not from right to left (giving numbers and to the carbon atoms at which branches are attached). C  C  C  C  C  C  C  C  C C C  C C  C  C  C  C  C  C  C  C C C  C . The names of alkyl groups attached as a branch are then prefixed to the name of the parent alkane and position of the substituents is indicated by the appropriate numbers.

If different alkyl groups are present, they are listed in alphabetical order. Thus, name for the compound shown above is: -ethyl- - methylnonane. [Note: the numbers are The name of such branched chain alkyl group is placed in parenthesis while naming the compound. While writing the trivial names of substituents’ in alphabetical order, the prefixes iso- and neo- are considered to be the part of the fundamental name of alkyl group.

The prefixes sec - and tert - are not considered to be the part of the fundamental name. The use of iso and related common prefixes for naming alkyl groups is also allowed by the IUPAC nomenclature as long as these are not further substituted. In multi- substituted compounds, the following rules may aso be remembered: If there happens to be two chains of equal size, then that chain is to be selected which contains more number of side chains. After selection of the chain, numbering is to be done from the end closer to the substituent.

-( , -Dimethylpropyl)nonane Problem . Structures and IUPAC names of some hydrocarbons are given below. Explain why the names given in the parentheses are incorrect. -Ethyl- , -dimethylcyclohexane (not -ethyl- , -dimethylcyclohexane) Cyclic Compounds : A saturated monocyclic compound is named by prefixing ‘ cyclo ’ to the corresponding straight chain alkane.

If side chains are present, then the rules given above are applied. Names of some cyclic compounds are given below. -sec-Butyl- -isopropyldecane (a) Lowest locant number, , , is lower than , , , (b) substituents are in equivalent position; lower number is given to the one that comes first in the name according to alphabetical order. , , - Trimethyloctane [ and not , , -Trimethyloctane ] -Ethyl- -methylheptane [ and not -Ethyl- -methylheptane ] .

. N o m e n c l a t u r e o f O r g a n i c Compounds having Functional Group(s) A functional group, as defined earlier, is an atom or a group of atoms bonded together in a unique manner which is usually the site of -( -Ethylbutyl)- , -dimethyldecane [and not -( , -Dimethylbutyl)- -ethyldecane] chemical reactivity in an organic molecule. Compounds having the same functional group undergo similar reactions. For example, CH OH, CH CH OH, and (CH ) CHOH — all having -OH functional group liberate hydrogen on reaction with sodium metal.

The presence of functional groups enables systematisation of organic compounds into different classes. Examples of some functional groups with their prefixes and suffixes along with some examples of organic compounds possessing these are given in Table . . First of all, the functional group present in the molecule is identified which determines the choice of appropriate suffix.

The longest chain of carbon atoms containing the functional group is numbered in such a way that the functional group is attached at the carbon atom possessing lowest possible number in the chain . By using the suffix as given in Table . , the name of the compound is arrived at. In the case of polyfunctional compounds, one of the functional groups is chosen as the principal functional group and the compound is then named on that basis.

The remaining functional groups, which are subordinate functional groups, are named as substituents using the appropriate prefixes. The choice of principal functional group is made on the basis of order of preference. The order of decreasing priority for some functional groups is: -COOH, –SO H, -COOR (R=alkyl group), COCl, -CONH , -CN,-HC=O, >C=O, -OH, -NH , > C=C<, -C ≡ C- . The –R, C H -, halogens (F, Cl, Br, I), –NO , alkoxy (–OR) etc.

are always prefix substituents. Thus, a compound containing both an alcohol and a keto group is named as hydroxyalkanone since the keto group is preferred to the hydroxyl group. For example, HOCH (CH ) CH COCH will be named as -hydroxyheptan- -one and not as -oxoheptan - -ol. Similarly, BrCH CH=CH is named as -bromoprop- -ene and not -bromoprop- -ene.

If more than one functional group of the same type are present, their number is indicated by adding di, tri, etc. before the class suffix. In such cases the full name of the parent alkane is written before the class suffix. For example CH (OH)CH (OH) is named as ethane– , –diol.

However, the ending – ne of the parent alkane is dropped in the case of compounds having more than one double or triple bond; for example, CH =CH-CH=CH is named as buta– , –diene. Problem . Write the IUPAC names of the compounds i-iv from their given structures. The functional group present is an alcohol (OH).

Hence the suffix is ‘-ol’. The longest chain containing -OH has eight carbon atoms. Hence the corresponding saturated hydrocar- bon is octane. The -OH is on carbon atom .

In addition, a methyl group is attached at th carbon. Hence, the systematic name of this compound is -Methyloctan- -ol. The functional group present is ketone (>C=O), hence suffix ‘-one’. Presence of two keto groups is indicated by ‘di’, hence suffix becomes ‘dione’.

The two keto groups are at carbons and . The longest chain contains carbon atoms, hence, parent hydrocarbon is hexane. Thus, the systematic name is Hexane- , -dione. Table .

Some Functional Groups and Classes of Organic Compounds Here, two functional groups namely ketone and carboxylic acid are present. The principal functional group is the carboxylic acid group; hence the parent chain will be suffixed with ‘oic’ acid. Numbering of the chain starts from carbon of – COOH functional group. The keto group in the chain at carbon is indicated by ‘oxo’.