Chapter: Introduction to Organic Chemistry: Alkanes

Transcription

1 Chapter: Introduction to Organic Chemistry: Alkanes Organic Chemistry An organic compound is a compound made from carbon atoms. has one or more C atoms. has many H atoms. may also contain O, S, N, and halogens. Typical organic compounds have covalent bonds. have low melting points. have low boiling points. are flammable. are soluble in nonpolar solvents. are not soluble in water. Organic vs. Inorganic Propane, C3H8, is an organic compound used as a fuel. NaCl, salt, is an inorganic compound composed of Na + and Cl ions. Why is propane organic, but NaCl is not? Writing Formulas for Alkanes: In organic compounds carbon has 4 valence electrons and hydrogen has 1. to achieve an octet, C forms four bonds. Tetrahedral Structure of Carbon VSEPR theory predicts that a carbon atom with four single, covalent bonds has a tetrahedral shape. Organic Molecules valence electrons are shared. covalent bonds form between carbon atoms. Tetrahedral Structure of Carbon In molecules with two or more carbon atoms, each carbon atom with four single bonds has a tetrahedral shape. Alkanes Structural Formulas Alkanes are written with structural formulas that are expanded to show each bond. condensed to show each carbon atom and its attached hydrogen atoms. Structural Formulas Condensed formulas are written for expanded structural formula by showing each carbon and the attached hydrogen atoms.

2 Names of Alkanes The names of alkanes are determined by the IUPAC (International Union of Pure and Applied Chemistry) system. end in ane. with 1 4 carbons in a chain use prefixes as follows. Name # Carbons Structural Formula Methane 1 CH4 Ethane 2 CH3CH3 Propane 3 CH3CH2CH3 Butane 4 CH3CH2CH2CH3 Names of Alkanes Alkanes with 5 10 carbon atoms in a chain use Greek prefixes. Name # Carbons Structural Formula Pentane 5 CH3CH2CH2CH2CH3 Hexane 6 CH3CH2CH2CH2CH2CH3 Heptane 7 CH3CH2CH2CH2CH2CH2CH3 Octane 8 CH3CH2CH2CH2CH2CH2CH2CH3 Nonane 9 CH3 CH2 CH2CH2CH2CH2CH2CH2CH3 Decane 10 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 Writing Structural Formulas Carbon atoms in a chain maintain tetrahedral shape. are connected in a zig zag pattern. are drawn as 2 dimensional. Can be written in several conformations. Hexane is an alkane with six carbon atoms in a continuous chain. has a zig zag look because each carbon atom is at the center of a tetrahedron. is represented by a ball and stick model as shown below. Cycloalkanes are cyclic alkanes. have two hydrogen atoms fewer than the open chain. are named by using the prefix cyclo before the name of the alkane chain with the same number of carbon atoms. The structural formulas of cycloalkanes are usually represented by geometric figures, Cyclopropane Cyclobutane Cyclopentane Cyclohexane

3 Alkanes with Substituents Isomers of Butane Isomers have the same molecular formula. have different atom arrangements. of butane (C4H10) are a straight chain and a branched chain. Alkyl groups are alkanes that are missing one H. substituents attached to carbon chains. named with a yl ending. Naming Substituents In the IUPAC system, a carbon branch is named as an alkyl group. halogen atoms are named as halo. Steps for naming Alkanes STEP 1 Name the longest continuous chain. STEP 2 Number chain. STEP 3 Locate substituents and name. Naming Cycloalkanes with Substituents The name of a substituent is placed in front of the cycloalkane name. methylcyclobutane chlorocyclopentane Properties of Alkanes Alkanes are nonpolar. insoluble in water. less dense than water. flammable in air. Alkanes with 1 4 carbon atoms are methane, ethane, propane, and butane. gases at room temperature. used as heating fuels. Alkanes with 5 8 carbon atoms are liquids at room temperature. pentane, hexane, heptane, and octane.

4 very volatile. used to make gasoline. Alkanes with 9 17 carbon atoms are liquids at room temperature have higher boiling points. are found in kerosene, diesel, and jet fuels. Alkanes with 18 or more carbon atoms have high molar masses. are waxy solids at room temperature. used in waxy coatings of fruits and vegetables. Reactions of Alkanes: Combustion In combustion reactions, alkanes react with oxygen. CO2, H2O and energy are produced. Alkane + O2 CO2 + H2O + heat Elements in Organic Compounds In organic molecules, carbon atoms bond with four bonds. mostly with H and other C atoms. sometimes to O, N, S, sometimes to halogens F, Cl, and Br.

5 Summary of Functional groups to know by the end of organic section are: a characteristic feature of organic molecules that behave in a predictable way. composed of an atom or group of atoms. groups that replace a hydrogen atom in the corresponding alkane. a way to classify families of organic compounds. Alkenes contain a double bond between adjacent carbon atoms. Alkynes contain a triple bond. An alcohol contains the hydroxyl ( OH) functional group. In an ether, an oxygen atom is bonded to two carbon atoms. An aldehyde contains a carbonyl group (C=O), which is a carbon atom with a double bond to an oxygen atom. In a ketone, the carbon of the carbonyl group is attached to two other carbon atoms. Carboxylic acids contain the carboxyl group, which is a carbonyl group attached to a hydroxyl group. An ester contains the carboxyl group between carbon atoms. In amines, the functional group is a nitrogen atom. In amides, the hydroxyl group of a carboxylic acid is replaced by a nitrogen group.