organic compounds alkanesand cycloalkanes

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    3. Organic Compounds:Alkanes and Cycloalkanes

    Based on

    McMurrys Organic Chemistry , 6th

    edition, Chapter 3

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    Families of Organic Compounds

    Organic compounds can be grouped into familiesby their common structural featuresWe shall survey the nature of the compounds in atour of the families in this course

    This chapter deals with alkanes , compounds thatcontain only carbons and hydrogens, allconnected exclusively by single bonds

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    3.1 Functional Groups

    Functional group - collection of atoms at a sitewithin a molecule with a common bonding patternThe group reacts in a typical way, generallyindependent of the rest of the moleculeFor example, the double bonds in simple and

    complex alkenes react with bromine in the same way(See Figure 3.1)

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    Survey of Functional Groups

    Table 3.1 lists a wide variety of functional

    groups that you should recognize (also insideback cover)As you learn about them in each chapter itwill be easier to recognize themThe functional groups affect the reactions,structure, and physical properties of everycompound in which they occur

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    Types of Functional Groups: Multiple

    CarbonCarbon Bonds* Alkenes have a C-C double bond* Alkynes have a C-C triple bond* Arenes have

    special bonds thatare represented asalternating singleand double C-Cbonds in a six-membered ring

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    Functional Groups with Carbon Singly Bonded

    to an Electronegative Atom* Alkyl halide: C bonded to halogen (C-X)

    * Alcohol: C bonded O of a hydroxyl group (C-OH)* Ether: Two Cs bonded to the same O (C-O-C)* Amine: C bonded to N (C-N)

    Thiol: C bonded to SH group (C-SH)S ulfide: Two Cs bonded to same S (C-S-C)Bonds are polar, with partial positive charge on C

    ( +) and partial negative charge ( ) onelectronegative atom

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    Groups with a CarbonOxygen Double Bond

    (Carbonyl Groups)* Aldehyde: one hydrogen bonded to C=O

    * Ketone: two Cs bonded to the C=O* Carboxylic acid: OH bonded to the C=O* Ester: C-O bonded to the C=OAmide: C-N bonded to the C=OAcid chloride: Cl bonded to the C=OCarbonyl C has partial positive charge ( +)Carbonyl O has partial negative charge ( -).

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    3.2 Alkanes and Alkane Isomers

    Alkanes: Compounds with C-C single bonds and C-Hbonds only (no functional groups)Connecting carbons can lead to large or small moleculesThe formula for an alkane with no rings in it must beCnH2n+2 where the number of Cs is n

    Alkanes are saturated with hydrogen (no more can beadded

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    Alkane Isomers

    CH 4 = methane, C 2H6 = ethane, C 3H8= propane

    The molecular formula of an alkane with more thanthree carbons can give more than one structureC4 (butane) = butane and isobutaneC

    5(pentane) = pentane, 2-methylbutane, and 2,2-

    dimethylpropane

    Alkanes with Cs connected to no more than 2 other Cs are straight-chain (continuous-chain ) or normal alkanesAlkanes with one or more Cs connected to 3 or 4 Csare branched-chain alkanes

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    Constitutional Isomers

    Isomers that differ in how their atoms are arranged inchains are called constitutional isomersCompounds other than alkanes can beconstitutional isomers of one another They must have the same molecular formula to be

    isomers

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    Condensed Structures of Alkanes

    We can represent an alkane in a brief form or inmany types of extended formA condensed structure does not show bonds but listsatoms, such as

    CH 3CH 2CH 3 (propane)

    CH 3C(CH 3)2CH 3 (2,2-dimethylpropane)

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    Names of Small Hydrocarbons

    No. of Carbons Formula Name (C n H2n +2)

    1 Methane CH 42 Ethane C 2H63 Propane C 3H84 Butane C

    4H

    105 Pentane C 5H126 Hexane C 6H147 Heptane C

    7H

    168 Octane C 8H189 Nonane C 9H20

    10 Decane C 10 H22

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    Problem 3.4 Five isomers of C 6H14 .

    3.5 a. two esters of C 5H10 O 2b two nitriles of C 4H7N

    3.6 How many isomersa. Alcohols of C 3H8Ob. Bromoalkanes of C

    4H

    9Br

    C10 H22 has 75 isomers

    C 20 H42 has over 360,000

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    3.3 Alkyl Groups

    Alkyl group remove one H from an alkane(a part of a structure)General abbreviation R (for Radical, anincomplete species or the rest of the

    molecule)Name: replace - ane ending of alkane with - yl ending

    CH 3 is methyl (from methane)CH 2CH 3 is ethyl from ethane

    See Table 3.4 for a list

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    You must know:

    One methyl groupOne ethyl groupTwo propyl groupsFour butyl groups (common names)

    iso- means methyl branch secondfrom far end of chain.

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    3.4 Naming Alkanes (IUPAC)

    Compounds are given systematic names by aprocess that uses

    Prefix-Parent-SuffixFollows specific rules

    Named as longest possible chain

    If two possibilities-chose one with more branchesCarbons in that chain are numbered in sequenceSubstituents are numbered at their point of attachment

    Compound name is one word (German style)Complex substituents are named as compounds wouldbe

    See specific examples in text

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    The IUPAC rules can be very complex,but each organic compound can be assigneda unique name, and from the name thestructure can be written. See appendix A.

    Many organic compounds are known bytheir common or trivial name especially

    if they are complex. These frequently give noclue as to the structure.

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    3.5 Properties of Alkanes

    Called paraffins (low affinity compounds)because they undergo few types of reactionsThey will burn in a flame, producing carbondioxide, water, and heat

    They react with Cl 2 in the presence of light toreplace Hs with Cls (not controlled)

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    Physical Properties

    Boiling points and melting points increase as size of alkane increasesForces between molecules (temporary dipoles,dispersion) are weak

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    Problems from Chapter 3Pages 93-101

    21-36, 39, 42-47, 49-54

    OWL for chap 3Sept 22

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    3.6 Cycloalkanes

    Cycloalkanes are alkanes that have carbon atomsthat form a ring (called alicyclic compounds)

    Simple cycloalkanes rings of CH2 units, (CH2) n ,or C n H2 n Structure is shown as a regular polygon with thenumber of vertices equal to the number of Cs (aprojection of the actual structure)

    cyclopropanecyclohexanecyclopentane

    cyclobutane

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    Complex Cycloalkanes

    Naturally occurring materials contain cycloalkane structuresExamples: chrysanthemic acid (cyclopropane),

    prostaglandins (cyclopentane), steroids (cyclohexanes andcyclopentane)

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    Properties of Cycloalkanes

    Melting points are affected by the shapes and theway that crystals pack so they do not change

    uniformly

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    3.7 Naming Cycloalkanes

    Count the number of carbon atoms in the ring and the number inthe largest substituent chain. If the number of carbon atoms inthe ring is equal to or greater than the number in the substituent,the compound is named as an alkyl-substituted cycloalkaneFor an alkyl- or halo-substituted cycloalkane, start at a point of attachment as C1 and number the substituents on the ring sothat the second substituent has as low a number as possible.Number the substituents and write the nameSee text for more details and examples

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    3 8 Ci T I i i

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    3.8 Cis-Trans Isomerism in

    CycloalkanesRotation about C-C bonds in cycloalkanes is limited by thering structure

    Rings have two faces and substituents are labeled as totheir relative facial positionsThere are two different 1,2-dimethyl-cyclopropaneisomers, one with the two methyls on the same side (cis)of the ring and one with the methyls on opposite sides(trans)

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    Stereoisomers

    Compounds with atoms connected in the same order but whichdiffer in three-dimensional orientation, are stereoisomersThe terms cis and trans should be used to specifystereoisomeric ring structuresRecall that constitutional isomers have atoms connected indifferent order

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