Chapter 2 Saturated Hydrocarbons

Chapter 2 Saturated

Hydrocarbons

General, Organic, and Biological Chemistry, Fifth EditionH. Stephen Stoker

Brroks/Cole Cengage Learning. Permission required for reproduction or display.

Prepared by:GIZEL R. SANTIAGO

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Chapter 2 Topics

• Organic and Inorganic Compounds• Bonding Characteristics of Carbon• Hydrocarbons and Hydrocarbon Derivatives• Alkanes: Acyclic Saturated Hydrocarbons• Structural Formulas• Alkane Isomerism• Conformations of Alkanes• IUPAC Nomenclature for Alkanes• Line-Angle Structural Formulas for Alkanes• Classification of Carbon Atoms

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Chapter 2 Topics

• Branched-Chain Alkyl Groups• Cycloalkanes• IUPAC Nomenclature for Cycloalkanes• Isomerism in Cycloalkanes• Sources of Alkanes and Cycloalkanes• Physical Properties of Alkanes and Cycloalkanes• Chemical Properties of Alkanes and Cycloalkanes• Halogenated Alkanes and Cycloalkanes

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Organic and Inorganic Compounds

Compounds obtained from living organisms were called organic compounds, and compounds obtained from mineral constituents of the earth were called inorganic compounds.

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Friedrick Wöhler heated an aqueous solution of two inorganic compounds, ammonium chloride and silver cyanate, and obtained urea (a component of urine).

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Organic chemistry is the study of hydrocarbons (compounds of carbon and hydrogen) and their derivatives. Inorganic chemistry is the study of all substances other than hydrocarbons and their derivatives.

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Bonding Characteristics of Carbon

Why does the element carbon form five times as many compounds as all the other elements combined?

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The unique ability to bond to each other in a wide variety of ways that involve long chains of carbon atoms or cyclic arrangements (rings) of carbon atoms.

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Hydrocarbons and Hydrocarbon DerivativesA hydrocarbon is a compound that contains only carbon atoms and hydrogen atoms. Thousands of hydrocarbons are known. A hydrocarbon derivative is a compound that contains carbon and hydrogen and one or more additional elements. Additional elements commonly found in hydrocarbon derivatives include O, N, S, P, F, Cl, and Br. Millions of hydrocarbon derivatives are known.

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Hydrocarbons and Hydrocarbon Derivatives

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Hydrocarbons may be divided into two large classes: saturated and unsaturated. A saturated hydrocarbon is a hydrocarbon in which all carbon–carbon bonds are single bonds. Saturated hydrocarbons are the simplest type of organic compound. An unsaturated hydrocarbon is a hydrocarbon in which one or more carbon–carbon multiple bonds (double bonds, triple bonds, or both) are present. In general, saturated and unsaturated hydrocarbons undergo distinctly different chemical reactions.

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Two categories of saturated hydrocarbons exist, those with acyclic carbon atom arrangements and those with cyclic carbon atom arrangements. The term acyclic means “not cyclic.” The following notations contrast simple acyclic and cyclic arrangements of six-carbon atoms.

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Alkanes: Acyclic Saturated Hydrocarbons

An alkane is a saturated hydrocarbon in which the carbon atom arrangement is acyclic. Thus an alkane is a hydrocarbon that contains only carbon–carbon single bonds (saturated) and has no rings of carbon atoms (acyclic). The molecular formulas of all alkanes fit the general formula CnH2n2, where n is the number of carbon atoms present. The number of hydrogen atoms present in an alkane is always twice the number of carbon atoms plus two more, as in C4H10, C5H12, and C8H18.

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Structural Formulas

The three simplest alkanes are methane (CH4), ethane (C2H6), and propane (C3H8). Three different methods for showing the three-dimensional structures of these simplest of all alkanes are given.

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Structural Formulas

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Two-dimensional structural representations for organic molecules are called structural formulas. A structural formula is a two-dimensional structural representation that shows how the various atoms in a molecule are bonded to each other.

Structural Formulas

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Structural formulas are of two types: expanded structural formulas and condensed structural formulas. An expanded structural formula is a structural formula that shows all atoms in a molecule and all bonds connecting the atoms.

Structural Formulas

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A condensed structural formula is a structural formula that uses groupings of atoms, in which central atoms and the atoms connected to them are written as a group, to convey molecular structural information.

Structural Formulas

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Structural Formulas

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Structural Formulas

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A skeletal structural formula is a structural formula that shows the arrangement and bonding of carbon atoms present in an organic molecule but does not show the hydrogen atoms attached to the carbon atoms.

Structural Formulas

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Isomers are compounds that have the same molecular formula (that is, the same numbers and kinds of atoms) but that differ in the way the atoms are arranged. Isomers, even though they have the same molecular formula, are always different compounds with different properties.

Alkane Isomerism

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There are two four-carbon alkane isomers, the compounds butane and isobutane. Both have the molecular formula C4H10.

Alkane Isomerism

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A continuous-chain alkane is an alkane in which all carbon atoms are connected in a continuous nonbranching chain. The other C4H10 isomer, isobutane, has a chain of three carbon atoms with the fourth carbon attached as a branch on the middle carbon of the three-carbon chain. It is an example of a branched-chain alkane. A branched-chain alkane is an alkane in which one or more branches (of carbon atoms) are attached to a continuous chain of carbon atoms.

Alkane Isomerism

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Alkane Isomerism

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Alkane Isomerism

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Alkane Isomerism

Constitutional isomers are isomers that differ in the connectivity of atoms, that is, in the order in which atoms are attached to each other within molecules.

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Conformations of Alkanes

Rotation about carbon–carbon single bonds is an important property of alkane molecules. Two groups of atoms in an alkane connected by a carbon–carbon single bond can rotate with respect to one another around that bond, much as a wheel rotates around an axle.

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A conformation is the specific three-dimensional arrangement of atoms in an organic molecule at a given instant that results from rotations about carbon–carbon single bonds.

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IUPAC Nomenclature for Alkanes

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Rule 1: Identify the longest continuous carbon chain (the parent chain), which may or may not be shown in a straight line, and name the chain.

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Rule 2: Number the carbon atoms in the parent chain from the end of the chain nearest a substituent (alkyl group).

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IUPAC Nomenclature for AlkanesRule 3: If only one alkyl group is present, name and locate it (by number), and prefi x the number and name to that of the parent carbon chain.

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IUPAC Nomenclature for AlkanesRule 4: If two or more of the same kind of alkyl group are present in a molecule, indicate the number with a Greek numerical prefix (di-, tri-, tetra-, penta-, and so forth). In addition, a number specifying the location of each identical group must be included. These position numbers, separated by commas, precede the numerical prefi x. Numbers are separated from words by hyphens.

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IUPAC Nomenclature for AlkanesRule 5: When two kinds of alkyl groups are present on the same carbon chain, number each group separately, and list the names of the alkyl groups in alphabetical order.

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IUPAC Nomenclature for AlkanesRule 5: When two kinds of alkyl groups are present on the same carbon chain, number each group separately, and list the names of the alkyl groups in alphabetical order.

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Rule 6: Follow IUPAC punctuation rules, which include the following: (1) Separate numbers from each other by commas. (2) Separate numbers from letters by hyphens. (3) Do not add a hyphen or a space between the last-named substituent and the name of the parent alkane that follows.

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1.Draw the condensed structural formula for 4,5-diethyl-3,4,5-trimethyloctane.2.Draw skeletal structural formulas for, and assign IUPAC names to, all C6H14 alkane constitutional isomers.3. Draw skeletal structural formulas for, and assign IUPAC names to, all C5H12 alkane constitutional isomers.

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Line-Angle Structural Formula for Alkanes

A line-angle structural formula is a structural representation in which a line represents a carbon–carbon bond and a carbon atom is understood to be present at every point where two lines meet and at the ends of lines.

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Structural Representations for Alkane Molecules

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Structural Representations for Alkane Molecules

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Classification of CarbonsEach of the carbon atoms within a hydrocarbon structure can be classified as a primary, secondary, tertiary, or quaternary carbon atom. A primary carbon atom is a carbon atom in an organic molecule that is bonded to only one other carbon atom. Each of the “end” carbon atoms in the three-carbon propane structure is a primary carbon atom, whereas the middle carbon atom of propane is a secondary carbon atom.

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Classification of Carbons

A secondary carbon atom is a carbon atom in an organic molecule that is bonded to two other carbon atoms.

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A tertiary carbon atom is a carbon atom in an organic molecule that is bonded to three other carbon atoms.

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A quaternary carbon atom is a carbon atom in an organic molecule that is bonded to four other carbon atoms.

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Branched-Chain Alkyl GroupsThere are various conformations of branched-chain alkyl groups. For example, these structures all represent an isopropyl group:

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Branched-Chain Alkyl Groups

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Branched-Chain Alkyl Groups

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Complex Branched Alkyl GroupsThe IUPAC system provision for such groups involves naming them as though they were themselves compounds. Select the longest alkyl chain in the complex substituent as the base alkyl group. The base alkyl group is then numbered beginning with the carbon atom attached to the main carbon chain. The substituents on the base alkyl group are listed with appropriate numbers, and parentheses are used to set off the name of the complex alkyl group.

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Complex Branched Alkyl Groups

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Cycloalkanes

A cycloalkane is a saturated hydrocarbon in which carbon atoms connected to one another in a cyclic (ring) arrangement are present. The simplest cycloalkane is cyclopropane, which contains a cyclic arrangement of three carbon atoms.

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Cycloalkanes

The general formula for cycloalkanes is CnH2n. Thus a given cycloalkane contains two fewer hydrogen atoms than an alkane with the same number of hydrogen atoms (CnH2n2). Butane (C4H10) and cyclobutane (C4H8) are not isomers; isomers must have the same molecular formula .

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Cycloalkanes

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Cycloalkanes

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Cycloalkanes

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Cycloalkanes

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Isomerism in Cycloalkanes

Constitutional isomers are possible for cycloalkanes that contain four or more carbon atoms.

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IUPAC Nomenclature for CycloalkanesIUPAC naming procedures for cycloalkanes are similar to those for alkanes. The ring portion of a cycloalkane molecule serves as the name base, and the prefix cyclo- is used to indicate the presence of the ring. Alkyl substituents are named in the same manner as in alkanes.

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IUPAC Nomenclature for Cycloalkanes

Numbering conventions used in locating substituents on the ring include the following: 1. If there is just one ring substituent, it is not necessary to locate it by number.

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2. When two ring substituents are present, the carbon atoms in the ring are numbered beginning with the substituent of higher alphabetical priority and proceeding in the direction (clockwise or counterclockwise) that gives the other substituent the lower number.

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3. When three or more ring substituents are present, ring numbering begins at the substituent that leads to the lowest set of location numbers. When two or more equivalent numbering sets exist, alphabetical priority among substituents determines the set used.

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Stereoisomers are isomers that have the same molecular and structural formulas but different orientations of atoms in space. Several forms of stereoisomerism exist.

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Cis–trans isomers are isomers that have the same molecular and structural formulas but different orientations of atoms in space because of restricted rotation about bonds.

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Cis is a prefix that means “on the same side.” Trans - is a prefix that means “across from.”

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Cis is a prefix that means “on the same side.” Trans - is a prefix that means “across from.”

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Isomerism in CycloalkanesUse of the terms cis- and trans- in designating stereoisomers in cycloalkanes is limited to substituted cycloalkanes in which the two substituted carbon atoms each have one hydrogen atom and one substituent other than hydrogen. The designations cis- and trans- become ambiguous in situations where either or both of the substituted carbons have two different substituents but no hydrogen atoms.

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Determine whether cis–trans isomerism is possible for each of the following cycloalkanes. If so, then draw structural formulas for the cis and trans isomers. a. Methylcyclohexane b. 1,1-Dimethylcyclohexane c. 1,3-Dimethylcyclobutane d. 1-Ethyl-2-methylcyclobutane

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Determine whether cis–trans isomerism is possible for each of the following cycloalkanes. If so, then draw structural formulas for the cis and trans isomers. a. 1-Ethyl-1-methylcyclopentane b. Ethylcyclohexane c. 1,3-Dimethylcyclopentane d. 1,1-Dimethylcyclooctane

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Sources of Alkanes and Cycloalkanes

Natural gas and petroleum (crude oil) constitute their largest and most important natural source. Deposits of these resources are usually associated with underground dome-shaped rock formations

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Physical Properties of Alkanes and Cycloalkanes

1. Alkanes and cycloalkanes are insoluble in water.

2. Alkanes and cycloalkanes have densities lower than that of water.

3. The boiling points of continuous-chain alkanes and cycloalkanes increase with an increase in carbon chain length or ring size.

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Physical Properties of Alkanes and Cycloalkanes

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Chemical Properties of Alkanes and Cycloalkanes

Alkanes are the least reactive type of organic compound. They can be heated for long periods of time in strong acids and bases with no appreciable reaction. Strong oxidizing agents and reducing agents have little effect on alkanes. Alkanes are not absolutely unreactive. Two important reactions that they undergo are combustion, which is reaction with oxygen, and halogenation, which is reaction with halogens.

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A combustion reaction is a chemical reaction between a substance and oxygen (usually from air) that proceeds with the evolution of heat and light (usually as a flame).

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A halogenation reaction is a chemical reaction between a substance and a halogen in which one or more halogen atoms are incorporated into molecules of the substance.

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A substitution reaction is a chemical reaction in which part of a small reacting molecule replaces an atom or a group of atoms on a hydrocarbon or hydrocarbon derivative.

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Note the following features of this general equation: 1. The notation R—H is a general formula for an alkane. R— in this case represents an alkyl group. Addition of a hydrogen atom to an alkyl group produces the parent hydrocarbon of the alkyl group.

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2. The notation R—X on the product side is the general formula for a halogenated alkane. X is the general symbol for a halogen atom. 3. Reaction conditions are noted by placing these conditions on the equation arrow that separates reactants from products. Halogenation of an alkane requires the presence of heat or light.

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Nomenclature of Halogenated Alkanes

A halogenated alkane is an alkane derivative in which one or more halogen atoms are present. Similarly, a halogenated cycloalkane is a cycloalkane derivative in which one or more halogen atoms are present.

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1. Halogen atoms, treated as substituents on a carbon chain, are called fluoro-, chloro-, bromo-, and iodo-.

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2. When a carbon chain bears both a halogen and an alkyl substituent, the two substituents are considered of equal rank in determining the numbering system for the chain. The chain is numbered from the end closer to a substituent, whether it be a halo- or an alkyl group.

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3. Alphabetical priority determines the order in which all substituents present are listed.

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Physical Properties of Halogenated Alkanes Halogenated alkane boiling points are generally higher than those of the corresponding alkane. An important factor contributing to this effect is the polarity of carbon–halogen bonds, which results in increased dipole–dipole interactions. Some halogenated alkanes have densities that are greater than that of water, a situation not common for organic compounds. Chloroalkanes containing two or more chlorine atoms, bromoalkanes, and iodoalkanes are all more dense than water.

End of Chapter 2 Saturated

Hydrocarbons

General, Organic, and Biological Chemistry, Fifth EditionH. Stephen Stoker

Brroks/Cole Cengage Learning. Permission required for reproduction or display.

Chapter 2 Saturated Hydrocarbons

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Transcript of Chapter 2 Saturated Hydrocarbons