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Lectures By: Ahmed Ali Hullio Organic Chemistry

NOTE: I typed some parts of this lecture. Please Type the remaining words of pasted papers of book. Dont type structures and figures

ALKANESAlkanes are open chain saturated hydrocarbons, methane (CH4) being the simplest member of this family of compounds. each carbon atom in alkanes is sp3 hybridized, and has tetrahedral geometry. The next member of this family ia ethane (C2H6) the formula of which is derived from that methane by replacing one of its hydrogen atom by methyl CH3 group. Similarly, the molecular formula of the next alkane, i.e., propane (C3H8), is obtained by replacing one of hydrogen atoms of ethane by methyl group. Similarly, the molecular formula of the next higher alkane, i.e, propane is obtained by replacing one of the hydrogen atoms of ethane by methyl group.In this way the molecular formulas of the higher alkanes can be derived from the lower ones by successively replacing hydrogens by methlyl groups. Thus, the next higher alkane is butane which is followed by pentane, hexane, and so on.We notice that the formula of each alkane differs from that of its neighbor by a unit of CH2 and all alkanes can be represented by a general formula ------ (Where n is the number of carbon atoms). A series of compounds that can be represented by a general formula is called a homologous series, and the individual members of the series are called homologs. Keeping in view the quadrivalency of carbon the univalency of hydrogen, we find that there is only one structure possible for each of methane, ethane and propane, as shown above. However, when a hydrogen atom of propane is replaced by methyl group, we obtain two different structures of butane----- depending on whether a hydrogen atom of the terminal carbon or that of the middle carbon of propane is replaced by methyl group. In the former case we get a straight chain structure of butane, whereas in later case we get a branched-chain structure, as shown above:

The straight-chain structure was called normal (n-) butane and the branched chain structure was called isobutene. In a similar way, we obtain three different structures of pentane ----- two of which are similar to those of butane, and are named as n-pentane and isopentane. The third (new) one is called neopentane.

In general, the prefix n- is used for a straight-chain compound, the prefix iso- is used for compounds with two methyl groups at the end of an otherwise straight chain, and the prefix neo- is employed to denote three methyl groups at the end of a chain. We have seen that although n-butane and isobutene have different structures, they have same molecular formula. Similarly, in spite of different structures, the three pentanes have the same molecular formula. The compounds that have same molecular formula but differ in structure are known as isomers. The number of possible isomers increases very rapidly with an increasing number of carbon atoms. For example, whereas there is only one methane, one ethane, and one propane, there are 2 butanes, 3 pentanes, 5 hexanes, 9 heptanes, 18 octanes, 35 nonanes and 75 decanes.

CYCLOALKANES

Cycloalkanes are aliphatic saturated hydrocarbons in which the sp3 hybridized carbon atoms are arranged in a ring and except for cyclopropane and cyclobutane all have tetrahedral geometry. These are represented by the general formula CnH2n which shows that these have two hydrogen atoms fewer than alkanes. In fact, two carbons of the molecule utilize one of their valence bonds each in the formation of the ring. If the molecular formula of a saturated hydrocarbon corresponds to the general formula CnH2n-2 the compound contains two rings in its molecule, and the general formula CnH2n-4 shows the presence of three rings in the molecule.

Since Cycloalkanes are aliphatic in nature and have cyclic structure, these are also commonly known as alicyclic compounds. These are also sometimes called carbocyclic compounds to differentiate them from heterocyclic compounds (compounds containing at least one atom other than carbon in their ring).

BICYCLIC COMPOUNDSTwo are more rings can be joined into bicyclic or polycyclic systems. There are three ways that two rings may be joined.

Fused bicyclic: Sharing two adjacent carbon atoms and the bond between them.Bridged bicyclic: A compound containing two rings joined at nonadjacent carbon atoms. Spirocyclic: In such compounds two rings share only one carbon atom.

Unsaturated Hydrocarbons

Unsatured hydrocarbons are those compounds of carbon and hydrogen which contain fewer than maximum number of hydrogen atoms and thus their chemical affinity is not fully satisfied. Instead, they have carbon-carbon multiple bonds and have a tendency to add reagents such as hydrogen, halogens, hydrogen halides, ets.. to form saturated compounds. Unsaturated hydrocarbons can be divided into tgwo main calsses, alkenes and alkynes.

ALKANES

Alkenes are the unsaturated hydrocarbons which contain a corbon-carbon double bond, and can be represented by the geneal formula CnH2n. They are also known as olefins, an old term derived from the Lain words Oleum meaning Oil and ficare meaning to make, because the lower-molecualr-weight gaseous alkenes form derivatives which are oily in appearnace. The carbon-carbon double bond is thus also called olefinic bond. Many compounds contain more than one carb-carbon double bond. They are known as alkadienes if the number of double bonds is two, alkatrienes for three double bonds, alkatetraenes for four double bonds, etc. Whereas the hydrocarbons with one carbon-carbon double bond are represented by the general formula CnH2n, those with two double bonds have the general formula CnH2n, with three double bonds, CnH2n-4, and so on. The location of each double bond in the molecule is indicated by appropriate number, as explained in nomenclature. However, they are classified according to their relative positions in the molecule, which influence the properties of the compounds. If two double bonds are adjacent, i.e, they have one carbon atom common, they are known as cummulated double bonds, e.g. ---------------------------------

The compounds with cumulated doulbe bonds are generally called allenes. The allenes are highly reactive compunds and therefore are diffucult to prepare. This is why only few allenes are known. If the two double bonds are separated by a single bond, they are called conjugated double bonds, e.g..

--------------------------Conjugation of doulbe bonds profoundly affects the physical and chemical properties of compounds. If the two double bonds are separated by two or more single bonds, they are called isolated double bonds, e.g,;

----------------------------

The isolated double bonds act essentially independently of each other. The term alkene is specifically used for those unsaturated hydrocarbons which have isolated carbon-carbon double bonds.

Certain suitable alkenes exist as cis-trans stereoisomers called Geometric isomers

ALKYNESAlkynes are the unsaturated hydrocarbons that contain a carbon-carbon triple bond in their molecules. A triple bond is comprised of one strong bond and two weaker bonds. It total strength is therefore greater than that of a double bond; it is also shorter than the double bond. The alkynes are preprsented by general formulas CnH2n-2. Acetylene (C2H2), is member of this family. The C-C triple bond is therefore known as acetylenic bond.

Internal and Terminal Alkynes

Carbon-carbon triple bond at 1,2-position in sufficiently larger chain is called internal alkynes. Whereas triple bond at any position other than 1,2-position is called terminal alkynes.

Akyl Halides

THE FUNCTIONAL GROUPOrganic molecules, in general, consist or two parts, a carbon skeleton and a functional group. The carbon skeleton is generally present in the form of univalent alkyl groups (or aryl groups) that are formed by the removal of one hydrogen atom from alkanes. It mainly consists of carbon-carbon single bonds and carbon-hydrogen bonds and is relatively much less reactive than the other part of the molecule. If undergoes the reactions typical of alkanes which do not have any functional group that may be a primary (pri-), a secondary (sec-), or a tertiary (tert-) alkyl group, depending on whether its carbon atom with an incomplete valence is attached to one, two or three other carbon atoms, respectively. Some of the alkyl groups of different types are illustrated below:--------------------------It is necessary to identify the alkyl group for the purpose of naming any organic compunds. The other part of the organic molecules, i.e, the functional group, mainly (but not always) consists of the heteoatoms (atoms other than carbon and hydrogen) and is responsible for the chemical properties (and many of the physical proerties) of the organic compounds. It is this part of the molecule where most of the chemical reactions occur. It also provides the structural basis on which the vast number of organic compunds can be classified into a relatively small number of families. Thus, a functional group is an atom of a group of atoms that defines the structure of a particular family of organic compunds, and also determines their properties. For examplel, the functinal group of an alkene is its carbon-carbon double bond andn that of an alkyne is its carbon-carbon triple bond. As we have seen chapter 5, most of the chemical reactions of alkenes and alkynes are due to these carbon-carbon multiple bonds. Some othe other common functinal groups of various calsses of organic compounds are shown in parentheses: organohalogen compunds (--X, where X represents F, Cl, Br or I), alcohols and phenols (--OH), ethers (--O--), aldehydes (--CHO), ketones (--CO--), carboxylic acids (--COOH), and amines (--N--).

A large part of organic chemistry is essentially the chemistry of various functional groups of if a compound contains more than one functional group, its chemistry is expected to be roughly composite of the chemistry of all groups. The properties of one functional group may be, of course, modified by the presence of another group in the same molecule. It is, therefore, important to consider and understand the effect of one functional group on another. However, we shall confine ourselves mainly to the individual functional groups and , in our subsequent studies, develop their chemistry in terms of their structures, nomenclature, properties and preparation from and conversion of other functional groups.

ORGANOHALOGEN COMPOUNDS Compunds in which one or more halogen atoms are bonded to carbon are known as organohalogen compounds. The functinal group in these compounds is the halogen atom and the reactions that are characteristic of this family are due to the halogen atom. Since these are generally the halogen derivatives of alkanes, these are also called haloalkanes and may be mono-, di-, tri-, or in general, polyhaloalkanes, depending on the number of halogen atoms present in the molecule. Monohaloalkanes are usually called alkyl halides and are classified according to the nature of the alkyl group to which the halogen atom is attached. For example

It should noted that similar names do not always mean the same classification. For example, isopropyl chloride is a secondary alkyl halide, whereas isobutyl iodide is a primary alkyl halide. Dihaloalkanes are classified according to the relative positions of the halogen atoms. Removal of two hydrogen atoms from the same carbon atom of an alkane gives an alkylidene group. Thus, if the two halogen atoms are attached to the same carbon atom, i.e, in the general (gem-) position, the dihaloalkane (gem-dihalide) is named as an alkylidene dihalide, e.g,.

------------------------------------------()If the two halogen atoms are on adjacent carbon atoms, i.e,, in the vicinal (vic-) position, the dihaloalkane (vic-dihalide) is named as a dihalide of the alkene from which it may be prepared by the addition of halogen, e.g..

--------------------()When the halogen atoms are on the terminal carbon atoms of the chain, the compound is name as a polymethylene dihalide, e.g..

These were the common names which are used only in limited cases of relatively simple haloalkanes. These compounds with two halogen atoms at relative positions other than those mentioned above, with two different halogen atoms, or with

Alcohols, Phenols and Ethers

Alcohols, phenols and ethers all may be regarded and derivatives of water. This is the reason they are being discussed together. Alcohols and phenols are derived from water b y replacing one of its hydrogen by alkyl and aryl groups, respectively.

------------------------------()Thus, both alcohols and phenols have the hydroxyl (--OH) functional group which is responsible for most of their chemistry. In alcohols are hydroxyl group is attached to an aliphatic carbon atom, while in phenols the hydroxyl group is directly attached to an aromatic nucleus. Thus, alcohols and phenols may also be regarded as the hydroxyl derivatives of aliphatic and aromatic hydrocarbons, respectively. Ethers are derived from water by replacing both of is hydrogens by two alkyl, two aryl, or one alkyl and one aryl groups. ---------------------------()The geometry of alcohols and ethers is similar to that of water, with oxygen being sp3 hybridized. However, van der Walls repulsion involving alkyl groups than in alcohol: di-tert-butyl ether is an extreme case -----------------------------()In opoxides where oxygen atom is a part of three-membered ring, the COC bond angle is highly distorted from the normal tetrahedral value. In ethylene oxide, e.g.; the bong angle of oxygen is 61.5. In phenol, the COH angle (109) is almost tetrahedral.

ALCOHOLSAlcohols are substances having the hydroxyl group attached to an aliphatic carbon atom. They may be classified according to the number of hydroxyl groups present in their molecules. An alcohol having only one hydroxyl group in its molecule is known as monohydric alcohol, and the alcohols with two and three hydroxyl groups in their molecules are known as dihydric and trihydric alcohols, respectively. The alcohols having more than one hydroxyl group are generally called polyhydric alcohols.

MONOHYDRIC ALCOHOLSMonohydric alcohols are represented by the general formula CnH2n+1 OH, and form a homologous series. They may be classified as primary, secondary, and tertiary alcohols, depending on whether the hydroxyl group is attached to a primary, secondary, or tertiary alkyl group, respectively.

--------------------()The structural differences lead to important differences in the chemical behavior of these alcohols.

POLYHYDRIC ALCOHOLS

Compounds containing more than one hydroxyl group attached to the aliphatic carbon atom in their molecules are generally known as polyhydric alcohols. The most common of the polyhydric alcohols are di- and trihydric alcohols that two and three hydroxyl groups, respectively, in their molecules.

----------------------------()The 1,2-diols are also commonly known as glycols and their names are derived from the alkenes from which they may be prepared by hydroxylation. For example,

------------------------------()The most important dihydric alcohol is ethylene glycol which is also simply known as glycol.The trihydric alcohols, in the IUPAC system, are named as triols. The only important trihydric alcohol is propane-1,2,3-triol which is commonly known as glycerol.

PHENOLSPhenols as a class are the compounds in which one or more hydroxyl groups are directly attached to a benzenoid aromatic ring including the polycyclic aromatic ring system. Any number of this class of compounds is generally known as a phenol. The word phenol is also used as the name of a specific compound, hydroxybenzene

Nomenclature of Phenols. Phenols being the specific name of the parent compounds, hydroxybenzene, it is used as a parent name is most of its derivatives. --------------------------------------()While naming the polyfunctional aromatic compounds, the hydroxyl function is generally placed low in the order of precedence; only the amino and the ether functions are placed lower than the hydroxyl function. -----------------------()

Many of the phenol derivates are know by their common named, as shown below. Their systematic names are also given in parentheses. -----------------------------()

ETHERS

Ethers are compounds in which an oxygen atom is bonded to two carbon atoms like COC; such linkage is known as ether linkage. Ethers may be regarded as dialkyl, alkyl aryl or diaryl derivatives of water.

-------------------------------()When two alkyl or aryl groups are the same, the ether is said to be symmetrical, e.g.; diethyl ether, C2H5OC2H5. Diethyl ether is also called simply ether, and is well known for its use as an anaesthetic and as a solvent. When the two alkyl (or aryl) groups are different, the ether is said to be unsymmetrical or mixed ether, e.g,. ethyl methyl ether, C2H5OC2H5; the same is true for alkyl aryl ether. E.g, methyl phenyl ether. CH3OC6H5. The general formula of the aliphatic ethers is CnH2n+2O which is the same as for the monohydric alcohols, but the value of n in this case is not less than 2.

CYCLIC EHTERCyclic ethers are those compounds which have their oxygen atom as part of a ring. The chemistry of ether linkage is essentially the same whether it forms part of an open chain or an aliphatic ring. Thus, in their preparation and properties, most cyclic ethers are just like the ethers we have already studied. Since divalent oxygen has bond angles not very different from those of carbon, the rings of cyclic ethers can exit in much the same conformations are the cycloalkane rings, i.e.. they can be puckered and, if small, can be strained. The parent member of cyclic ethers is ethylene oxide in which oxygen is part of the three-membered ring. The three-membered rings containing oxygen are commonly known as epoxides. One of the most important of the cyclic ethers is tetrahydrofuran (THF) which we have encountered frequently as a solvent. While numbering the cyclic ethers, oxygen atom is given the number 1. -----------------------------()

It is not unusual to hve more than one ether linkage in a ring, e.g., 1,4-dioxane. 1,4-Dioxane is another useful solvent for many organic reactions. Molecules containing several ether linkages are referred to as polyethers. Cyclic polyethers containing 4 or more ether linkages in a ring of 12 or more atoms are called crown ethers, because their molecular models resemble crowns, .e.g, 12 crown-4.

Epoxides and crown ethers deserve special attention because of their unusual properties. Epoxides:Epoxides are three-membered cyclic ethers. Because they are readily prepared from alkenes, they are generally named as alkene oxides, e.g,; ethylene oxide. Substitutive IUPAC nomenclature names epoxides as epoxy derivatives of alkanes, e.g; ethylene oxide becomes epoxyethane. The prefix epoxy- always immediately precedes the name of the alkane, without being included in the alphabetical order of the other substituents. According to the formal IUPAC nomenclature, they are called oxiranes-------------------().

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ALDEHYDES AND KETONESAldehydes and ketones are the compounds containing carbonyl group, (---------), in their molecules. They can be regarded as the derivatives of formaldehyde, H2C==O, an aldehyde itself, that is obtained by the oxidation of methane. Formaldehyde is the simplest carbonyl compounds. If one of the two hydrogen atoms of formaldehyde is replaced by an alkyl or aryl group, another aldehyde is obtained. If both hydrogens are replaced by either alkyl or aryl groups, or one by each, a ketone is obtained. --------------------------------------()Thus aldehydes are represented by the general formula RCHO (in formaldehyde, R is H). the characteristic functional group of aldehyde is (----------) which is known as the formyl group. The aldehyde function (the formyl group) always occur at the end of a chain. The ketones are represented by the general formula RCOR where R and R may be aliphatic or aromatic; they may be same or different. The characteristic functional group of ketones is (-------) which commonly known as the keto group. It always occurs inside a chain.

Carboxylic Acids and Their DerivativesCarboxylic acids are the organic compounds containing carboxyl group, --COOH, in their molecules. The carboxyl group is so named because it is a combination of carbonyl and hydroxyl groups. The compounds having the carboxyl group are known as carboxylic acids because they are markedly acidic in nature. Although the carboxyl function is a combination of the carbonyl and the hydroxyl group, the properties of each of the two functional groups are so modified by the presence of the other that the carboxyl group behaves a entirely a new functional group. The carboxylic acids may be represented by the general formula, RCOOH, where R may be hydrogen an aliphatic or an aromatic group. The aliphatic carboxylic acids are often referred to as fatty acids because many of these can be obtained from the hydrolysis of the animal and vegetable fats. In fact, the designation aliphatic for open-chain organic compounds arose from this early use of fats as a source of such compounds. The term fatty acids applies more specifically to the naturally occurring higher aliphatic carboxylic acids e.g,; palmitic acid (C15H31COOH), stearic acid (C17H35COOH), oleic acid (C17H33COOH) and linoleic acid (C17H31COOH), which occur as glycerides, the esters of glycerol.

A compound can contain more than one carboxyl groups e.g. oxalic acid, malonic acid and succinic acid. Carboxyl group when attached directly to aromatic ring will be aromatic carboxylic acid e.g. Benzoic acidACID DERIVATIVES

ACID CHROLIDESAcid chlorides are the substances which are formed by replacing OH of the carboxyl group by CL. Since they contain chlorine attached to an acyl group, they are also known as acyl chlorides. Though other acid halides are also known, acid chlorides are most common.The names of the acid chlorides are derived from the named of the corresponding carboxylic acids by replacing the ending ic of the name of the parent acid by yl, and replacing the word acid by the word chloride. Remember that the o of the ending oic is retained. For example, -----------------------------------------()

ACID ANHYDRIDESAcid anhydrides are the substances which are obtained after the elimination of a water molecule form the carboxyl groups of two carboxylic acid molecules (or form the two carboxyl groups of dicarboxylic acids). They are regarded as diacyl oxides, just as ethers are regarded as dialkyl oxides. The names of the simple acid anhydrides (obtained from two molecules of the same acid) are derived from the named of the corresponding carboxylic acids by replacing the word acid by the word anhydride. For mixed acid anhydrides (obtained from two different acids), the parent name of each acid is written in alphabetical order. Followed by the word anhydride. ----------------------()The lower-molecular weights anhydrides are liquid at room temperature. The anhydrides above nonanoic anhydride are solid. The boiling points of anhydrides are close to those of aldehydes, ketones and other unassociated compounds of comparable molecular weights. Anhydrides are generally soluble in common organic solvents.

ESTERSEsters are the substances which are obtained after the elimination of water molecule between an acid and an alcohol.Since esters are regarded as formed from an acid and an alcohol, their names are derived by writing the name of the alkyl group of the alcohol, followed by the name of the acid with the ending ic acid replaced by ate. For example,----------------------()If the ester function (--COOR ) is to be treated as a substituent, it is designated as a carboalkoxy group.

AMIDESAmides are the substances in which OH of the carboxyl group has been replaced by NH2 group. The names of amides are derived by replacing ic acid of the common names or the oic acid of the IUPAC names of the parent carboxylic acids by amide, for example,

------------------------()The amide of benzoic acid is called benzamide.

ORGANIC NITRO COMPOUNDS

There is a very large number of organic compounds that contain nitrogen in their molecules. In these compounds nitrogen mostly forms a part of their functional group. The chemistry of the nitrogenous organic compounds varies very widely because of the variety of oxidation states available to nitrogen. The nitrogen-containing functional groups are of varying importance in organic chemistry; the most important of these are amines.

AMINESAmines are derivatives of ammonia in which one are more hydrogen atoms have been replaced by alkyl or aryl groups. Amines are classified as primary, secondary or tertiary depending on the number of hydrogen atoms replaced in the ammonia molecule. --------------------------------------()In secondary and tertiary amines the alkyl or aryl groups may be the same or different. The designations primary, secondary and tertiary are used here to denote the degree of substitution on nitrogen, not the nature of the alkyl groups.

Amino group if attached directly to aromatic ring is called aromatic amine e.g. Aniline. If a compound contains nitrogen as a part of close carbon chain is called heterocyclic compound.

NOTE: Acid anhydride, ester and amide can occur in cyclic form are called cylic anhydride, latone and lactam.

Aromatic Hydrocarbons The term aromatic derived from the Greek work aroma meaning fragrant, was originally used in organic chemistry for a special class of compounds, particularly those having a low hydrogen-to-carbon ratio in their molecular formula, that have a characteristic odour. However, it was soon realized that many aromatic compounds are odourless, whereas many other organic compounds are fragrant though they are not aromatic. Later, it was observed that most of the aromatic compounds have a six-carbon unit in their molecules, line benzene, and that this unit it retained through most chemical transformations. Furthermore, when aromatic compounds of higher molecular weight were subjected to various methods of degradation, they often produced benzene or a derivative of benzene. Benzene was therefore recognized as the parent member of this class of compounds. In the mean time, it was realized that benzene, the simplest aromatic compound, has characteristic structural features due to which it shows special behavior. The work aromatic has thus come to designate characteristic structural features, rather than odour, and all compounds that are structurally related to benzene are termed as aromatic. The aromatic hydrocarbons, therefore, include benzene and those compounds of carbon and hydrogen that resemble benzene in their chemical behavior. It appears from our definition of aromatic hydrocarbons that any study of this class of compounds must begin with a study of benzene. In order to understand the chemistry of benzene, it is necessary to have an understating of is structure.

Polycyclic benzenoid aromatic compounds.In addition of the monocyclic aromatic compounds, polycyclic benzenoid compounds (consisting of two or more fused benzene rings) such as naphthalene, anthracene, phenanthrene, pyrene, etc,. also show aromatic properties. These fused ring system, in general, contain a Huckel number of () electron. For example, naphthalene has ten (--) electrons, and anthracene and phenanthrene have fourteen electrons each. However, pyrene has sixteen electrons which is not a Huckel number, but still it shown the aromatic properties. It has been suggested that in the case of the fused-ring system, the Huckels rule should be applied to the peripheral electrons. In this way, pyrene that has fourteen peripheral electrons, also obeys the Huckel (4n+2) electrons rule.

POLYCYCLIC AROMATIC HYDROCARBONSAromatic hydrocarbons containing two or more benzene rings in their molecules are called polycyclic aromatic hydrocarbons. They may be divided into two main classes. (1) those in which the benzene rings are isolated, e.g, biphenyl, terphenyls (ortho, meta and para), and diphenylmethane. --------------------()(2) those in which the benzene rings are fused together at ortho positions so that the adjacent rings have a common carbon-carbon bond, e.g, naphthalene, anthracene and phenanthrene. -------------------------()The former class of polycyclic aromatic hydrocarbons may be considered as phenyl-substituted benzenes and methanes, and are treated like the other substituted benzenes and methanes discussed earlier. We shall therefore confine ourselves to the latter class of the polycyclic aromatic hydrocarbons, i.e;, the fused-ring aromatic hydrocarbons.

NONBENZENOID AROMATIC COMPOUNDSThese compounds will be discussed under the topic of aromaticity and Huckle rule.

HETEROCYCLIC COMPOUNDS

Term Hetero means: Different These are cyclic compounds which contain other atom as a part of carbon close chain. Other atom i.e. hetero atom can be oxygen, nitrogen or sulphur. Heterocyclic compounds may contain more than one same or different atoms as a part of cyclic chain. Heterocyclic compounds can be aromatic or aliphatic

Classification of organic compounds

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