Post on 16-Nov-2018
Organic Chemistry - Introduction
Introduction
Organic chemistry is the study of the covalent chemical compounds that contain primarily Carbon and Hydrogen. Organic molecules can also contain small amounts of the nonmetallic elements Fluorine, Chlorine, Bromide, Iodine, Oxygen, Nitrogen and/or Sulfur.
Organic chemistry is a large branch of chemistry. For example, there are about 1,500,000 known inorganic compounds with new compounds being discovered or synthesized at a rate of about 10,000 per year. Within the discipline of organic chemistry there are more than 19,000,000 known compounds and new compounds are being discovered or synthesized at a rate of over 200,000 per year.
Why are there so many different organic compounds?
1. Carbon is one of the few elements that can form stable covalent bonds with itself; with other carbon atoms.
2. Carbon must form four covalent bonds to satisfy the octet rule. These bonds can be any mixture of single bonds, double bonds, or triple bonds to satisfy the octet rule.
3. Carbon can form stable covalent bonds with other non-metals, such as the halogens, hydrogen, oxygen, nitrogen, and sulfur.
4. The covalent bonds with the other non-metals can be single, double, or triple bonds, provided the other element can share sufficient electrons to form these bonds and at the same time satisfy the octet rule.
Organic compounds are divided into two major classes and the major classes are divided into numerous subclasses and groups. The division into classes, subclasses, and groups is based upon specific arrangements of atoms within the molecule that result in a characteristic set of physical and chemical properties. These specific arrangements of atoms are called FUNCTIONAL GROUPS.
The first major class of organic molecules is the HYDROCARBONS. This class of molecules, as the name suggests, is composed of only hydrogen and carbon.
The HYDROCARBON class of compounds is divided into two main subclasses; the ALIPHATIC HYDROCARBONS and the AROMATIC HYDROCARBONS. Within the ALIPHATIC HYDROCARBON subclass there is three groups; the ALKANES, ALKENES, and ALKYNES.
ALKANES contain only carbon & hydrogen and all the chemical bonds in these molecules are single bonds. The alkanes are also called the SATURATED HYDROCARBONS. In these molecules all of the chemical bonds between carbon atoms are single bonds. This arrangement of only carbon-carbon single bonds in the molecule allows for the maximum amount of hydrogen to be bonded to the carbon; the molecules are saturated with hydrogen.
ALKENES contain one or more carbon-carbon double bonds. The functional group within this group of molecules is the carbon-carbon double bond.
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ALKYNES contain one or more carbon-carbon triple bonds.
AROMATIC HYDROCARBONS contain a PHENYL RING (BENZENE RING); a cyclic arrangement of 6 carbon atoms and eighteen electrons that results in unique physical and chemical properties.
The ALKENES, ALKYNES, and the AROMATIC HYDROCARBONS are called the UNSATURATED HYDROCARBONS because they contain less than the maximum amount of hydrogen. Hydrogen can be added to these molecules reducing the carbon-carbon double bonds, carbon-carbon triple bonds, or the phenyl ring system to carbon-carbon single bonds.
The second major class of organic compounds is the SUBSTITUTED HYDROCARBONS. These compounds contain primarily carbon and hydrogen with one or more non-metallic element present in the molecule. Substituted Hydrocarbons can be divided into four subclasses.
The first subclass is the ORGANOHALIDES. These compounds contain one or more Halogen atoms (Fluorine, Chlorine, Bromide or Iodine) bonded to a carbon atom. When the halogen is attached to an open chain compound the molecule is a ALKYL HALIDE, when the halogen is bonded to an aromatic compound the molecule is an ARYL HALIDE.
The second subclass is the OXYGEN containing organic molecules. This is the largest and most complex subclass of substituted hydrocarbons. Within this subclass there are four major groups of compounds;
ALCOHOLS ALDEHYDES & KETONES CARBOXYLIC ACIDS.
The compounds in these groups react with themselves or with each other to form seven subgroups of compounds;
Alcohols react with alcohols to form ETHERS Alcohols react with Aldehydes to form HEMIACETALS and ACETALS Alcohols react with Ketones to form HEMIKETALS and KETALS Alcohols react with Carboxylic Acids to form ESTERS Carboxylic acids react to form CARBOXYLIC ACID ANHYDRIDES.
The SULFUR containing subclass contains one major group of compounds, the THIOLS. Thiols sometimes go by the name MERCAPTAN. The Thiols react with themselves to form THIOETHERS and DISULFIDES, and they react with carboxylic acids to form THIOESTERS.
The primary subclass of substituted organic molecules that contain NITROGEN is the AMINES. Amines can be made to react with carboxylic acids to form AMIDES.
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C C
C C
C C
C Halogen
C OH
C O C
R1 CO
H
R1 C R2
R1 C
OH
O R2H
R1 C
OH
O
R2
R3
R1 C
O
O R2H
R3
R1 C
O
O
R2
R3
R4
R1 CO
OH
R1 CO
O R2
CO
C
O O
R2R1
C SH
C S C
C S S C
R1 CO
S R2
C N
R1 CO
N
Alkanes
Alkenes
Alkynes
Aromatics
Alkyl Halides
Alcohols
Ethers
Aldehydes
O
Ketones
Hemiacetals
Hemiketals
Acetals
Ketals
Carboxylic Acids
Esters Amides
Amines
Thiolesters
Disulfides
Thiolethers
Thiols
Carboxylic AcidAnhydrides
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Organic Chemistry - The Alkanes
THE HYDROCARBONS
The Hydrocarbon class of organic compounds are composed of only carbon and hydrogen, hence their name. Within Hydrocarbons there are two subclasses, the Aliphatic Hydrocarbons and the Aromatic Hydrocarbons. The Aliphatic Hydrocarbon subclass is divided into three groups, the Alkanes, Alkenes and Alkynes. The Alkanes and Alkenes are found in biochemistry; the Alkynes are rarely if ever found in living organisms.
THE HYDROCARBONS - THE ALKANES
The Alkanes are saturated hydrocarbons. They contain only carbon and hydrogen and all of the covalent bonds between the atoms are single bonds. In Alkanes, all non carbon-carbon bonds are carbon-hydrogen bonds. The molecule is said to be saturated with hydrogen atoms.
The first 10 straight chain or normal (n) alkanes are:
H3C
H2C
CH3
H3C
H2C
CH2
CH3
H3C
H2C
CH2
H2C
CH2
CH3
H3C
H2C
CH2
H2C
CH3
C
H
H
H
H
H C
H
C
H
H
H
H
Methane
Ethane C2H6
C3H8Propane
Pentane
C4H10
C6H14
C5H12
Hexane
Butane
CH4
NameEmpiricalFormula
(Condensed) Structural Formula
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The straight chain alkane molecules all fit the general formula CnH2n+2.
Carbon can form stable covalent bonds with other carbon atoms, as witnessed by the list of straight chain alkanes. This chemical property of carbon allows for the formation of branched chain alkanes as well. Structural formulas for three alkanes are depicted below, two of them (black) are branched chain alkanes:
Both of the branched chain molecules have an empirical formula of C10H22, the same molecular formula as decane (red). It is obvious that these three molecules are all different. They have different structures which means that they have different physical and chemical properties. Organic molecules that have identical empirical formulas, but different structural formulas are called CONSTITUTIONAL ISOMERS or STRUCTURAL ISOMERS. The branched chained molecules depicted above are two of several constitutional isomers of decane.
Carbon atoms within straight chain, branched chain, or cyclic alkane (see below) can be designated as primary (1°), secondary (2°), tertiary (3°), or quaternary (4°) carbon atoms based upon the number of carbon-carbon bonds in which they are involved. A primary carbon (1° carbon) is directly bonded to one, and only one other carbon. Primary carbon atoms are the terminal (end) carbons. Secondary carbon atoms are directly bonded to two other carbon atoms; tertiary carbons are bonded to three other carbons; and quaternary carbons are involved in four carbon-carbon bonds.
H3C
H2C
CH2
H2C
CH2
H2C
CH2
H2C
CH2
CH3
H3C
H2C
CH2
H2C
CH2
H2C
CH2
CH3
H3C
H2C
CH2
H2C
CH2
H2C
CH3
H3C
H2C
CH2
H2C
CH2
H2C
CH2
H2C
CH3
C10H22Decane
Heptane
Octane
Nonane
C7H16
C8H18
C9H20
H3C
H2C
CH2
H2C
CH2
H2C
CH2
H2C
CH2
CH3
H3CCH C
H2
CH C
H2
CH CH3
CH3 CH3 CH3H3C C
HC C
HCH3
CH3 CH3 CH3
CH2
CH3
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Nomenclature
Formation of branched chain organic molecules greatly increases the number of possible organic molecules and it greatly complicates the nomenclature of organic molecules. Since CONSTITUTIONAL ISOMERS are obviously different molecules, a system of nomenclature must be in place to uniquely name all of the possible organic molecules. The system of nomenclature was devised by the International Union of Pure and Applied Chemistry (IUPAC). Before the rules for naming alkanes can be applied, the names and structures of some of the common branches, side chains, or substituent groups need to be presented. These branches are called ALKYL GROUPS.
CH3
H2C CH3
H2C
H2C CH3
H2C
H2C
H2C CH3
H2C
H2C
H2C
H2C CH3
CH
CH3
CH3
H2C CH
CH3
CH3
HC
CH3
H2C CH3
C
CH3
CH3
CH3
methyl
ethyl
propyl
butyl
pentyl
isopropyl
isobutyl
sec-butyl
tert-butyl
Name Condensed Structural Formula
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IUPAC Rules for Naming Alkanes
5. All alkanes end in “ane”. 6. Find and identify the parent compound; the longest continuous chain of carbon atoms. Make sure to
examine (count) all of the different possible conformations, from all possible directions. 7. Name the parent chain using the names for the straight chain (normal) alkanes. 8. Name each branch attached to the parent chain using the names of the common alkyl groups (above). 9. Attach the name of the alkyl group to the name of the parent alkane as a prefix. 10. When two or more groups are attached to the parent chain, the names the substituted alkyl groups are
listed in alphabetical order. 11. When two or more substituents are identical, use prefixes such as di (2), tri (3), tetra (4), penta (5),
etc., and specify the location of each group by a number. These prefixes DO NOT affect the alphabetical order of the substituted alkyl groups.
12. The carbon atoms of the parent chain are numbered starting from whichever end of the chain that results in the location of the first branch having the lowest possible number. The first substituent group must always have the lowest possible number. If two numbering schemes are available and the first group in each scheme has the same number, then the lowest combination of numbers is employed.
13. When two or more groups are attached to the parent each is located with a unique number. 14. When identical groups are on the same carbon, repeat the numbers locating this group in the name. 15. Numbers are separated from names by hyphens. 16. Numbers are separated from each other by commas.
Some examples:
H3CH2C
HC CH3
CH3
2-methylbutane
H3CH2C
HC
H2C CH3
CH2
CH3
3-ethylpentane
H3CH2C C
H
HC
H2C CH3
CH2
CH3
CH2
CH2
CH3
3,4-diethylheptane
H3C CH
HC C
H
H2C
H2C CH3
CH3
CH
H3C CH3
CH2
CH3
4-ethyl-3-isopropyl-2-methylheptane
H3C CH
H2C
HC
H2C C
H
H2C
HC CH3
CH3
CH3
CH3
CH3
2,4,6,8-tetramethylnonane
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CYCLIC ALKANES
Constitutional isomers form because carbon can form stable bonds with itself. One of the consequences of the chemistry of carbon is that the end of a linear alkane molecule can be made to react with the other end to form a cyclic compound. When the cyclic compounds contain only carbon and hydrogen and when all of the carbon-carbon bonds are single bonds then the resulting cyclic compounds are CYCLIC ALKANES. All alkanes with three or more carbon atoms can form cyclic compounds. Cyclic alkanes with five or six carbons are the most stable because the angles of these regular geometric forms, pentagons and hexagons, most closely approximate the bond angles of tetrahedral carbon atoms. All cyclic alkane molecules fit the general formula CnH2n. Several representations of the four most common cycloalkanes, from the most expanded to the most condensed structure, are given below. The most condensed form is used most often.
H3C CH
C CH
CH3
CH3H3C CH3
CH2
CH3
3-ethyl-2,3,4-trimethylpentane
H3CH2C C
H
HC C
H
H2C
H2C C CH3
CH2
CH3
CH3
CH2
CH3
CH
H3C CH3
CH3
6-ethyl-7-isopropyl-3,3,8-trimethyldecane
H3C CH
H2C
HC C
H
H2C
HC C CH3
CH2
CH3
CH3
C
CH3
CH2
CH3H3C
CHH3C CH3
CH2
CH3
CH3
5-tert-butyl-7-ethyl-4-isobutyl-2,8,8-trimethyldecane
H3C C
CH3
CH3
C
CH2
CH3
CH2
CH3
C
CH2
CH3
CH2
CH3
C
CH3
CH3
CH3
3,3,4,4-tetraethyl-2,2,5,5-tetramethylhexane
C
C CH
H
H H
H
H
H2C
H2C CH2
C
C C
C
H
H H
H
H
H
H
H
H2C CH2
CH2H2C
cyclobutane
cyclopropane
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Nomenclature
1. All of the rules for naming straight chain alkanes apply to cyclic alkanes. 2. Cyclic alkanes all contain the prefix “CYCLO” immediately preceding the name of the parent alkane. 3. If only one group is substituted on the cyclic alkane then the number “1” can be omitted from the
name. If more than one substituent group is present then each must be given a number and the carbon atom of the ring that carries the most complex group is given the number “1”. The most complex group is the group with the greatest moleculer mass.
4. If the cyclic alkane is part of a larger, more complex molecule then it is named as a substituent group; i.e., the cyclopropyl group, cyclobutyl group, cyclopentyl group, cyclohexyl group, etc.
Some examples:
C
C C
CC
H
H
HH
H
H
H
H
H
H
H2C
H2C CH2
CH2
H2C
C
C C
C
CCH
H
H
H
H
H H
H
H
H
H
H H2C
H2C
H2C CH2
CH2
CH2
cyclohexane
cyclopentane
CH3
methycyclobutane
CH3
CH3
1,2-dimethycyclobutane
H3C CH3
1,1-dimethylcyclopropane
H3C H2C
CH3
CH3
1-ethyl-2,4-dimethylcyclopentane
C
CH3
H3C
CH3
CH3
H2C
CHH3C CH3
CH3
1-tert-butyl-4-ethyl-2-isopropyl-5-methylcyclohexane
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Physical Properties
1. The alkanes are nonpolar molecules because there is little difference between the electronegativities of carbon and hydrogen.
2. Since the alkanes are nonpolar they are insoluble in water. Water is very polar. 3. Alkanes are soluble in nonpolar solvents. Nonpolar solvents include the hydrocarbons and alkyl
halides (see below). 4. The alkanes are less dense than water and will float on water. 5. Alkanes are nonpolar molecules with little attractive force between them. For this reason alkanes
with 1 to 4 carbons are gases at room temperature, those with 5 to 16 carbons are liquids, and when there are 17 carbons or more the alkanes are waxy solids.
6. At a fixed number of carbon atoms, as the number of branches (side chains) increase the melting points and boiling points of the constitutional isomers decrease. Branched chain alkanes do not fit together as well, do not pack into regular arrays as easily as straight chain molecules. More energy must be removed from branched chained molecules before they can condense into liquids or freeze into solids.
Chemical Reactions
Alkanes undergo two types of chemical reactions, COMBUSTION and SUBSTITUTION.
Combustion
Combustion is the complete oxidation of organic molecules to CO2 and H2O with the liberation of energy. An example of a combustion reaction is:
CH4 + 2O2 → CO2 + 2H2O + 213 kcal
All organic molecules undergo combustion reactions and this type of reaction is important to biochemistry.
Substitution
In a substitution reaction one atom or group of atoms on the molecule is replaced by another, different group. Alkanes and cycloalkanes undergo substitution reactions with the halogens, usually chlorine or
H3CH2C C
H
H2C
HC
H2C
HC
CH2
CH2
CH3
CH3
3-cyclopropyl-7-cyclohexyl-5-methyldecane
H3CH2C C
H2C
HC C
H
H2C C CH3
CH2
CH3
CH3
CH3
CH3
HC
H3C CH3
7-cyclohexyl-7-ethyl-4-isopropyl-2,2,5-trimethylnonane
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bromine. In this reaction a halogen atom is substituted for one of the hydrogen atoms on the alkane. The products of this reaction are an ALKYL HALIDE and a hydrogen halide. Halogenation can only occur in the presence of heat or light as a catalyst. In organic reactions, reaction conditions and/or catalysts are written over the arrow.
General reaction:
Specific examples:
Note: Organic reactions are seldom balanced because very often more than product is possible as demonstrated in the above example.
Alkyl Halide Nomenclature
1. All of the rules for naming alkanes apply to naming the alkyl halides. 2. When the halogens are a substituent group they are abbreviated fluoro (F), chloro (Cl), bromo (Br),
and/or iodo (I). 3. By convention halogen substituents are placed in the name before the alkyl substituents.
Specific examples:
C
X
C
H
X2 HX+ +
Lightor
Heat
Cl2 HCl+ +
Lightor
HeatC H
H
H
H
H C
Cl
H
H
chloromethane
Br2 HBr+ +Light
H C C
H
H
HH
H
H C C
H
H
BrH
H
bromoethane
Cl2 HCl+ +Heat
H C C C H
HHH
H H H
H C C C H
HHH
H H Cl
H C C C H
HHH
H Cl H
+
1-chloropropane 2-chloropropane
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H3CH2C
HC
HC
HC
H2C
HC CH3
CH3FClBr
6-bromo-5-chloro-4-fluoro-2-methyloctane
H3C CH2C C
H2C C
H2C
H2C CH3
Br Br
ClCH3
CH3
CH3
2,6-dibromo-6-chloro-2,2,4-trimethylnonane
Cl
Cl
Br
H2C CH3
CH3
1-bromo-1,5-dichloro-3-ethyl-2-methylcyclohexane
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