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3. Organic Compounds:Alkanes and Cycloalkanes

Based onMcMurry’s Organic Chemistry, 7th edition, Chapter 3

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Families of Organic Compounds

Structural features that make it possible to classifycompounds by reactivity are call functional groups

Organic compounds can be grouped into families by theircommon structural features

Chapter 3 deals with structure of some common functionalgroups but emphasizes alkanes and alkane isomers

Alkanes are compounds that contain only carbons andhydrogens, all connected exclusively by single bonds

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3.1 Functional Groups

A functional group is a collection of atoms at a sitewithin a molecule with a common bonding pattern

The group reacts in a typical way, generallyindependent of the rest of the molecule

For example, a double bond in simple and/or complexalkene reacts with bromine in the same way; see below

Br

Br

CH3 CH3Br

Br

CH3 CH3

starting material

Br2

starting material

Br2

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Survey of Functional Groups

Know Table 3.1 which lists a wide variety offunctional groups (also inside back cover)

As you learn about them in each chapter andin laboratory (i.e., the FTIR experiment) it willbe easier to recognize them

The functional groups affect the reactions,structure, and physical properties of everycompound in which they occur

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Types of Functional Groups: MultipleCarbon–Carbon 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 Bondedto an Electronegative Atom

*Alkyl halide: C bonded to halogen (C-X) * Alcohol: C bonded O of a hydroxyl group (C-OH) * Ether: Two C’s bonded to the same O (C-O-C) * Amine: C bonded to N (C-NR2; where R=H or C) Thiol: C bonded to SH group (C-SH) Sulfide: Two C’s bonded to same S (C-S-C) Bonds are polar, with partial positive charge on C (δ

+) and partial negative charge (δ−) on electronegativeatom; remember F O N Cl Br I S C H

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Groups with a Carbon–Oxygen Double Bond(Carbonyl “C=O” Groups)

*Ketone: two C’s bonded to the carbonyl, C=O *Aldehyde: one hydrogen bonded to C=O *Carboxylic acid: OH bonded to the C=O *Ester: C-O bonded to the C=O Amide: C-N bonded to the C=O Acid halide: X bonded to the C=O Carbonyl C has partial positive charge (δ+) Carbonyl O has partial negative charge (δ-).

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amide

ester

ester

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3.2 Alkanes and Alkane Isomers Alkanes: Compounds with C-C single bonds and C-H

bonds only (no functional groups) Connecting carbons can lead to large or small molecules The formula for an alkane with no rings in it must be

CnH2n+2 where the number of C’s is n Alkanes are saturated with hydrogen (no more can be

added Catentation -the formation of networks (often “chains”) of

identical atoms bond together

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Names of Small Hydrocarbons

C10H22decane10C9H20nonane9C8H18octane8C7H16heptane7C6H14hexane6C5H12pentane5C4H10butane4C3H8propane3C2H6ethane2CH4methane 1

(CnH2n+2)Formula NameNo. of Carbons

7535189532111

Possible alkane isomers“catenation”

C20H42isocane20 366,319

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Constitutional Isomers

Isomers that differ in how their atoms are arranged inchains are called constitutional isomers

Compounds other than alkanes can beconstitutional isomers of one another

They must have the same molecular formula to beisomers

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Alkane “Constitutional” Isomers

Constitutional Isomers - Same formula differentstructure; C5H12

straight-chain (continuous-chain) or normalalkanes

branched-chain alkanes

2,2-dimethylpropane2-methylbutanen-pentane

2,2-dimethylpropane2-methylbutanen-pentane

2,2-dimethylpropane2-methylbutanen-pentane

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Condensed Structures of Alkanes

A condensed structure does not show bondsbut lists atoms, such as isomers of pentane CH3(CH2) 3CH3 (n-pentane) (CH3) 2CH2CH2 CH3 (2-methylbutane) CH3C(CH3)2CH3 (2,2-dimethylpropane)

2,2-dimethylpropane2-methylbutanen-pentane

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3.3 Alkyl Groups

Alkyl group – remove one H from an alkane(a part of a structure)

General abbreviation “R” (the “rest” of themolecule and should be defined, for example,R=H, CH3)

Name: replace -ane ending of alkane with -ylending CH3 is “methyl” (from methane, Me-)

CH2CH3 is “ethyl” (from ethane. Et-) See Table 3.4 for a list

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You must know your abbreviations:Me methyl groupEt ethyl groupn-Pr, i-Pr for the two propyl groups

Four butyl groups (common names)n-Bu, sec-Bu, i-Bu, t-Bu

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Special positions (handed out in class)

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Types of Alkyl groups

Classified by the connection site (See Figure 3.3) a carbon at the end of a chain (primary alkyl group) a carbon in the middle of a chain (secondary alkyl

group) a carbon with three carbons attached to it (tertiary alkyl

group)

1° 2° 3° 4°

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Draw all isomers for C6H14; give their line-angle formulas.

Identify all primary, secondary, tertiarycenters by writing in 1°, 2°, 3° next to eachcarbon

Practice problem

1°2°

2° 2°2°

1°1°

1°

3° 2°2° 1°

3° 3°

3°2° 2°1° 1°

1°

1°

1°

1°1°

1°1°

1° 1°

2°

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This problem is the ONLYproblem to be turned in next period Draw all seven isomers for C5H12O

give their line-angle formulas. Identify all primary, secondary, tertiary centers

by writing in 1°, 2°, 3° next to each carbon In each isomer, circle and name each alkyl

branch attached to an oxygen atom. For example

Obutyl methyl

1° 2°2° 1°

1°

Called methyl butyl ether

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3.4 Naming Alkanes (IUPAC)

Compounds are given systematic names by aprocess that uses Prefix-Parent-Suffix

Follows specific rules Named as longest possible chain If two possibilities-chose one with more branches Carbons in that chain are numbered in sequence Substituents are numbered at their point of attachment Compound name is one word (German style) Complex substituents are named as compounds would

be 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.

Many organic compounds are known bytheir common or trivial name – especiallyif 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 reactions

They will burn in a flame, producing carbondioxide, water, and heat

They react with Cl2 in the presence of light toreplace H’s with Cl’s (not controlled)

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Physical Properties

Boiling points and melting points increase as size ofalkane increases

Forces between molecules (temporary dipoles,dispersion) are weak

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3.6 Cycloalkanes Cycloalkanes are alkanes that have carbon atoms

that form a ring (called alicyclic compounds) Simple cycloalkanes rings of CH2 units, (CH2)n,

or CnH2n Structure is shown as a regular polygon with the

number of vertices equal to the number of C’s (aprojection of the actual structure)

cyclopropane cyclohexanecyclopentanecyclobutane

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Complex Cycloalkanes Naturally occurring materials contain cycloalkane structures Examples: 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 changeuniformly

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3.7 Naming Cycloalkanes Count the number of carbon atoms in the ring and the number in

the 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 cycloalkane

For an alkyl- or halo-substituted cycloalkane, start at a point ofattachment 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 name See text for more details and examples

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3.8 Cis-Trans Isomerism inCycloalkanes Rotation about C-C bonds in cycloalkanes is limited by the

ring structure Rings have two “faces” and substituents are labeled as to

their relative facial positions There are two different 1,2-dimethyl-cyclopropane

isomers, 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 which

differ in “fixed” three-dimensional orientation, are stereoisomers The geometric isomer terms “cis” and “trans” should be used to

specify stereoisomeric ring structures Recall that constitutional isomers have atoms connected in

different order

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