Organic Chemistry. Hydrocarbons. Introduction to Organic Molecules. The simplest organic molecules containing only carbon and hydrogen atoms

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1 Introduction to Organic Molecules Organic Chemistry: (1828) Friedrich Wohler first synthesized an organic compound from an inorganic source leading to the birth of Organic Chemistry. Organic chemistry is the study of carbon containing compounds except elemental carbon (diamond, graphite, coal), CO 2, CO, carbonates (CO 3 2- group) and cyanides (CN - group); Which are all considered inorganic molecules. Organic Chemistry Carbon has the ability to form long chains. Without this property, large biomolecules such as proteins, lipids, carbohydrates, and nucleic acids could not form. Carbon easily forms bonds with other non-metal atoms. The simplest organic molecules containing only carbon and hydrogen atoms Four basic types: Alkanes Alkenes Alkynes Aromatic hydrocarbons Hydrocarbons Molecular Representations We use different representations to describe organic molecules for different purposes. The Lewis structures of alkanes look like this. Also called structural formulas. Often not convenient, though The space-filling model shows the theoretical overlapping electron clouds of bonded atoms 1

2 so more often than not, condensed structural formulas are used. Alkanes But, most of the time, I use line structures Lines represent bonds connecting carbons. Since all carbons contain 4 bonding electrons, all available bonds are assumed to be hydrogen. Saturated hydrocarbons. Saturated with hydrogens. Only single bonds. Also called Paraffins Relatively unreactive; however, highly combustible. Alkanes can be separated from the complex hydrocarbon mixture present in petroleum. The separation into simpler mixtures is accomplished by means of a fractionating column called fractional distillation. IUPAC Names for the First Ten Continuous- Chain Alkanes. IUPAC refers to the standard system of chemical nomenclature set forth by the International Union of Pure and Applied Chemistry. The simplest alkane is methane (CH 4 ) the primary compound in natural gas. Ethane (C 2 H 6 ) a minor component of natural gas. Propane (C 3 H 8 ) a fuel for heating homes and cooking. Octane (C 8 H 18 ) a fuel used for automobile combustion Dodecadecane (C 20 H 42 ) solid wax used for making candles and as a lubricant All of these are straight-chained hydrocarbons (C n H 2n+2 ). Many structural isomers of these exist, known as branched alkanes. Isomerism Isomers: Compounds that have identical molecular formulas, but different arrangement of atoms. Structural isomers: (AKA Constitutional isomers) molecules having the same molecular formula but different arrangement of atoms and bonds 2

3 Number of Structural Isomers Possible for Alkanes of Various Carbon- Chain Lengths. Suffix: Tells what type of compound it is. Suffix: Tells what type of compound it is. Prefix: Tells what groups are attached to chain. Cycloalkanes Carbon can also form ringed structures. Five- and six-membered rings are most stable. Can take on conformation in which angles are very close to tetrahedral angle. Smaller rings are quite strained. Alkenes - Contain at least one carbon carbon double bond. - Unsaturated. Have fewer than maximum number of hydrogens. C n H 2n Properties Similar to alkanes Non-polar Insoluble in water Soluble in non-polar solvents Less dense than water 3

4 Table 12-2, p.388 Which isomer would you predict to be more stable, and why? Properties of Alkenes Alkynes Notice that geometric isomerism exists for alkenes when the double bond is not on a terminal carbon. To determine the cis-/trans- geometric isomerism, draw a line through the bond and identify same or different same different Contain at least one carbon carbon triple bond. Carbons in triple bond sp-hybridized and have linear geometry. Also unsaturated. C n H 2n-2 Structure also affects physical properties of alkenes. Alkynes Ethyne (commonly called acetylene) is the simplest alkyne and is used as a fuel for torches and in making plastics. Aromatics Cyclic hydrocarbons based on the benzene ring Many aromatic hydrocarbons are known by their common names. CH 3 NO 2 NO 2 CO 2 H NO 3 4

5 Properties of Aromatic Compounds Non-polar Insoluble in water Aromatic rings are relatively stable chemically and often remain intact during reactions. Benzene and toluene are widely used organic solvents. Can not be produced by the human body; therefore, must be part of our diet. Organic Functional Groups The chemically reactive group, or groups, on organic molecules where reactions tend to occur. Carboxylate salt - ate : :.... : - Na + Sodium methanate 5