Organic Chemistry CE 541
Basic Concepts from Organic Chemistry • Elements “All organic compounds contain CARBON in combination to one or more elements”
Properties (compared to inorganics) • usually combustible • generally have lower melting and boiling points • less soluble in water • existence of isomers • slow reactions • have high molecular weight • most of organic compounds can serve as a food for bacteria
Sources of Organic Compounds • Nature: sugars, starch, vegetable oils, animal oils • Synthetic: manufacturing processes • Fermentation: Alcohols, anti-biotics, and acids which are derived by the action of microorganisms upon organic matter.
The Carbon Atom Why many compounds of carbon? • carbon atom has four electrons to share • ability of carbon atoms to link together in a wide variety of ways: • Continuous Open Chain
Chain with Branches • In a Ring
The structural formula of organic compound may be simplified, for example: Can be written as: CH3-CH2-CH2OH Or simply as: CH3CH2CH2OH
Isomerism Isomers “are compounds that contain the same number of the same atoms in different arrangements” • Examples • (1)
(2) (3) (4) Each of the four compounds has its own physical and chemical properties
Hydrocarbons “Are organic compounds that contain only Carbon and Hydrogen elements”. The simplest hydrocarbon is methane (CH4). Methane is composed of four (4) hydrogen atoms and one carbon atom • Hydrogen has one valence electron • Carbon has four valence electron CH4 Condensed Expanded Structural Structural Formula Formula
Aliphatic Hydrocarbons Two Types (1) Saturated “Those in which adjacent carbon atoms are joined by a single bond”
(2) Unsaturated “Those in which adjacent carbon atoms are joined by more than one bond (at least two of the carbon atoms)”
Alkanes (Saturated) “are also known as paraffins”. Their general molecular formula is: CnH2n+2(for open chain) Hexane (6 C and 14 H)
CnH2n (for cycloalkanes) Cyclohexane (6 C and 12 H)
The simplest alkane compound is methane (CH4). Methane is: • gas • highly explosive • greenhouse gas
The First Ten Continuous-Chain Alkanes Molecular Formula Structural Formula IUPAC Name n CH4 CH4 Methane 1 C2H6 CH3-CH3 Ethane 2 C3H8 CH3-CH2-CH3 Propane 3 C4H10 CH3-(CH2)2-CH3 Butane 4 C5H12 CH3-(CH2)3-CH3 Pentane 5 C6H14 CH3-(CH2)4-CH3 Hexane 6 C7H16 CH3-(CH2)5-CH3 Heptane 7 C8H18 CH3-(CH2)6-CH3 Octane 8 C9H20 CH3-(CH2)7-CH3 Nonane 9 C10H22 CH3-(CH2)8-CH3 Decane 10
Butane has two isomers n-butane isobutane (2 methylpropane)
Pentane has three isomers Pentane isopentane (2-methylbutane) Neopentane (2,2-dimethylpropane)
As the number of carbon atoms in alkanes was increased, the number of constitutional isomers will also increase: • hexane has 5 isomers • heptane has 9 isomers • octane has 18 isomers • nonane has 35 isomers • decane has 75 isomers
How to draw constitutional isomers? Take hexane (C6H14) as an example (hexane has 5 isomers) Draw a carbon skeleton (no hydrogen atoms) using all of carbon in a continuous chain. Remove one carbon atom from the end of the chain and place it on another carbon atom so that the new skeleton differs from the previous carbon skeleton. Repeat this procedure until you exhaust all possibilities of relocating one carbon atom
If carbon atom was place on one of the end atoms This is similar to the skeleton shown above
Next, if necessary, remove two carbon atoms from the continuous chain skeleton in (1) and relocate them either as single carbon atoms or as a two carbon fragment on other atoms in the chain. Write all possible different skeletons. Continue this procedure, if needed, for three carbon atoms, till you have the number of constitutional isomers as given in the problem. Check all skeletons to be sure that they are all different.
Place hydrogen atoms on the carbon atoms in each skeleton, remembering that there are four (4) bonds to each carbon atom. The 5 isomers are: CH3-CH2-CH2-CH2-CH2-CH3 CH3-CH-CH2-CH2-CH3 CH3 CH3-CH2-CH-CH2-CH3 CH3 CH3 CH3-C-CH2-CH3 CH3 CH3-CH-CH-CH3 CH3 CH3
Physical Properties of Alkanes (Table 5-1) • colorless • odorless • quite soluble in water (especially those having 5 carbon atoms or more) • dissolve in many solvents • at room temperature • C1 to C5 are gaseous • C6 to C17 are liquids • C18 and above are solids
Homologous Series When series of compounds differ by a common increment (such as CH2 in the case of alkanes), the series is referred to as being homologous series. In such cases, a general formula can be established to express the compounds (in this case CnH2n+2)
Radicals When one hydrogen atom is dropped from the methane- series hydrocarbon, the –ane ending is dropped and a –yl is added Methan CH4 CH3- methyl Ethane C2H6 C2H5- ethyl
Nomenclature First consider the nomenclature of the Alkyl groups. Alkyl groups are derived by removal of one hydrogen atom from an alkane. Alkyl groups are named by replacing the –ane ending of the alkane (methane) by –yl (methyl) in the case of simpler hydrocarbons.
Methane CH4 CH3- Methyl Ethane CH3CH3 CH3CH2- Ethyl Propane CH3CH2CH3 CH3CH2CH2- Propyl Butane CH3CH2CH2CH3 CH3CH2CH2CH2- Butyl Pentane CH3CH2CH2CH2CH3 CH3CH2CH2CH2CH2 - Pentyl Hexane CH3CH2CH2CH2CH2CH3 CH3CH2CH2CH2CH2CH3- Hexyl
Simple Alkyl Groups and their Names The most common alkyl groups will be discussed.
Butyl Groups The next set of alkyl groups contains four carbons. That is why they are called butyl groups. There are four types of butyl groups each with a different combination of carbon chain and bonding point.
The IUPAC (International Union of Pure and Applied Chemistry) name for Alkanes are obtained using the following rules: 1. All alkanes have the ending –ane 2. The longest continuous chain of carbon atoms is determined, and this chain is used as the parent structure. The parent structure is pentane. If the chain is in the form of a ring, then the prefix cyclo- is used.
3. The carbon atoms in this chain are numbered by starting at the end that would give the lowest numbers to the group or groups attached to the parent structure. In the above example, the lowest numbers would be obtained by numbering from the left side (2 and 3 are lower than 3 and 4) 4. The group attached to parent structure, other than hydrogen, is given both name and number. Halogens attached to the parent structure are named ( halo— ) Cl is chloro Br is bromo I is iodo NO2 is nitor Alkyl groups are given their IUPAC names
5. If more than one of the same group appears as a substitute in a given molecule, then the prefixes di, tri, tetra, and penta are used to indicate the number of times this group appears (two, three, four, and five times) The position of these groups on the numbered parent structure is indicated in increasing numerical order. Each group must have a number to indicate its position on the parent structure
Since 2 methyl groups are attached to the parent structure and hence di is used (dimethyl). The groups are attached at the 2nd and 3rd positions, so we use (2,3-dimethyl). The comma ( , ) is used between numbers and the hyphen ( - ) is used between numbers and names. 6. If more than one type of group is attached to the parent structure, these groups are placed in alphabetical order in the name of the compound. di - , tri - , tetra - , sec - , tert – are ignored in determining the order The underlined letter is used to determine the alphabetical order in the following list of groups dichloro , trimethyl , isopropyl , sec-butyl , tert-butyl , tetraethyl
Alkenes (Olefins) They are aliphatic hydrocarbons that have the general molecular formula CnH2n for open-chain systems • The simplest alkene is the Ethylene (C2H4) • Alkenes are unsaturated hydrocarbons, since they contain fewer than the maximum number of hydrogen atoms in their general formula • Alkenes contain a double bond