1 Reyes

Chapter 12 Introduction to Organic Chemistry: Alkanes

12.1 The Nature of Organic Molecules What is Organic Chemistry? The study of organic compounds. Structure of Organic Compounds An organic compound

Is a compound made from carbon atoms (one or more C’s and many H’s). May also contain O, S, N, and halogens.

Carbon is tetravalent In carbon compounds

Carbon has 4 valence electrons and hydrogen has 1. • • C • H • •

To achieve an octet, C forms four bonds. CH4 , methane Count the four bonds: Organic Molecules may be Polar What type(s) of intermolecular interactions can be displayed by these molecules? CH3Cl CCl4 3D Shapes of Organic Molecules Remember the VSEPR theory! A central atom forming four bonds has a tetrahedral shape. Wedges are used for atoms pointing towards us. Dashed-lines are used for atoms pointing away from us.

12.2 Families of Organic Molecules: Functional Groups To study the whole, we look at the parts… (i.e. functional groups) What is a functional group?

A characteristic feature of organic molecules. An atom or group of atoms within a larger molecule that has a characteristic structure and chemical behavior.

(Memorize table 12.1 on page 365)

2 Reyes

H C C C C H

H

H

H

H H

H

H

C HH

H

H C C C H

H

H

H

H

C HH

H

C HH

H

H C C C C C H

H

H

H

H

H

H

H

H

H

H

H C H

H

H

H C C

H

H

H

H

H H C C C

H

H

H

H

H

H

H

CH4 CH3CH3

CH3 CH3

CH3CH2CH3

CH2 CH3CH3

Compounds containing C and H

Hydrocarbons (Containing only C and H)

Aliphatic Aromatic (Open chain and cyclic) (contain the benzene ring) Alkanes Alkenes Alkynes Cycloalkanes and Cycloalkenes

12.3 The Structure of Organic Molecules: Alkanes and Their Isomers In Alkanes We find single bonds only.

A straight-chain alkane has all its carbons connected in a row (we call it backbone). A branched-chain alkane has branches connected to the backbone.

Constitutional (or Structural) Isomers They are compounds with the same molecular formula, but different connectivity. These three compounds have the formula, C5H12, but their structures (connectivity) are different. As the number of C’s increases, the number of possible isomers increases exponentially. (There are 75 isomers for the formula C10H22, and 366,319 isomers for the formula C20H42.) Why are isomers important? They have different properties (sometimes dramatically different properties; see table 12.2, page 369.)

12.4 Drawing Organic Structures Types of Formulas Molecular formulas show the type and number of atoms of each element in the compound. Methane Ethane Propane CH4 C2H6 C3H8

Expanded structural formulas (ESF’s) show all bonds of all the atoms as lines. Condensed structural formulas (CSF’s) are written by giving the name and number of atoms bonded to a particular carbon right in front of every carbon.

3 Reyes

Prefix Parent Suffix

Where are susbtituents located?

How many carbons?

What family does the molecule belong to?

Line-Bond Formulas The ends and bends are carbons. We assume every carbon is bonded to as many hydrogen atoms as needed in order to give them four bonds. (Hydrogen atoms are never shown!) CH3‒CH2‒CH2‒CH2‒CH2‒CH3

12.5 The Shapes of Organic Molecules Conformers C-C single bonds are free to spin around and this leads to different 3D geometries (Conformers). Due to this rotation we may write the same compound in different ways, check this example: All these structures represent the same compound (same number of each type of atom, and same connectivity.) In fact, there are an infinite number of possible geometries.

Again, all these structures represent the same compound…, make no mistake! They are not isomers! How can you tell when two or more structures are isomers (different compounds) or conformers (same compound)? Naming is the key!

12.6 Naming Alkanes IUPAC System The names of alkanes

Are determined by the IUPAC (International Union of Pure and Applied Chemistry) system. In the IUPAC system for organic compounds, a chemical name has three parts: prefix, parent, and suffix.

Named by counting the number of C atoms and adding the family suffix -ane.

The first four members have common names. The family of alkanes forms what we call a homologous series; that is, all the members of the family form a sequence in which there is a regular difference in the molar mass. The equation that describes the homologous series of the alkanes family is:

CnH2n + 2 (Where “n” is the number of C atoms.)

4 Reyes

Here is the list of the first twenty members of the alkanes family. Memorize the names for the first ten.

# of C atoms Formula Name

1 CH4 Methane

2 CH3CH3 Ethane

3 CH3CH2CH3 Propane

4 CH3CH2CH2CH3 n-Butane

5 CH3CH2CH2CH2CH3 n-Pentane

6 CH3CH2CH2CH2CH2CH3 n-Hexane

7 CH3CH2CH2CH2CH2CH2CH3 n-Heptane

8 CH3CH2CH2CH2CH2CH2CH2CH3 n-Octane

9 CH3CH2CH2CH2CH2CH2CH2CH2CH3 n-Nonane

10 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 n-Decane

11 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3 n-Undecane

12 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3 n-Dodecane

13 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3 n-Tridecane

14 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3 n-Tetradecane

15 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3 n-Pentadecane

16 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3 n-Hexadecane

17 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3 n-Heptadecane

18 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3 n-Octadecane

19 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3 n-Nonadecane

20 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3 n-Eicosane

(The “n” as a prefix to the name stands for “normal”. It means we are talking about the straight chain alkane – no branches.) Substitution patterns Carbons can be: The R represents more C’s. Primary Secondary Tertiary Quaternary Alkyl group (Table 12.4, page 381): It is the part of an alkane that remains when a hydrogen atom is removed. Their names are given by replacing the –ane suffix (from their parent alkane) with –yl. Keep in mind that carbon atoms in single bonds rotate freely; therefore, the following structures – and line-bond formulas – may take a slightly different shape. You have to be able to identify these side chains (substituents or branches) in any organic molecule.

CH3 or methyl

CH2 or ethylCH3

CH2 or propylCH2CH3

CH3 CH

CH3

or Isopropyl

CH3 CH2 CH2 CH2or Butyl

CH3 CH

CH3

CH2or Isobutyl

CH3 CH2 CH

CH3

or sec-butyl(secondary butyl)

CH3 C

CH3

CH3

or tert-butyl or t-butyl(tertiary butyl)

5 Reyes

Branched-chain alkanes Can be named by following four steps:

STEP 1: Name the main chain. Find the longest continuous chain of carbons. (You have to be careful here…, remember free rotation? Make sure you get the longest chain possible.)

The longest continuous chain has five carbons, not four (the straight chain)!

STEP 2: Number the carbon atoms in the main chain. Begin at the end nearer the first branch point: We want to give the lowest possible numbers to the branches.

STEP 3: Identify the branching substituents, and number each according to its point of attachment to the main chain:

A methyl is attached to C2. Both, a methyl and an ethyl are attached to C3.

STEP 4: Write the name as a single word. hyphens separate numbers from letters commas separate numbers if two or more different substituent groups are present, cite them in alphabetical order. for identical substituents use numeral prefixes.

2-methylpentane 3-ethyl-3-methylhexane 3,3-dimethylhexane Presence of Halogens (Haloalkanes) When halogens are present as substituents, they receive the following names: F- (fluoro), Cl- (chloro), Br- (bromo), and I- (iodo) Cl CH3

3-chloro-2-methylpentane

CH3─CH2─CH─CH─CH3 5 4 3 2 1 (Alkyl halides is a common name used for simple molecules.)

6 Reyes

What if…? 1. Two different substituents are found at the same distance from both ends? What if…? 2. In the same molecule there’s more than one possible option for the longest chain? Remember… In the name there must be as many numbers as substituents in the molecule. Practice Problem on Naming What’s the name of the following molecule?

12.7 Properties of Alkanes Properties of Alkanes

Odorless or mild odor. Colorless. Tasteless. Nontoxic. Have low melting points. (Why?) Have low boiling points. (Why?) Flammable. Soluble in nonpolar solvents. Insoluble in water. Not very reactive.

methylethyl

12

12

33

CH

CH

CH2

CH2

CH2

CH

CH

CH3 CH2

CH3

CH3CH2

CH3

CH

CH3 CH3

CH3

C CH2 C C CH2 CH3

CH3

CH3

CH3

CH3

F

F

F

CH3 C

CH3

CH2 CH2 CH3

CH

CH2 CH2 CH3

CH2 CH3

7 Reyes

Properties of Alkanes At room T (n-alkanes):

First four are gases. With 5-15 C’s are liquids. >16C’s are waxy solids.

Properties of Alkanes The boiling and melting points for the straight-chain alkanes increase with molecular size. WHY? Boiling Points of Alkanes

Are among the lowest of organic compounds(Why?).

Increase with larger molecules (Why?).

Decrease for branched alkanes (Why?).

12.8 Reactions of Alkanes Combustion of Alkanes Alkanes Undergo combustion by reacting with oxygen to produce carbon dioxide, water, and energy. alkane + O2 → CO2 + H2O + energy Balancing a Combustion Equation Write the equation C5H12 + O2 → CO2 + H2O Balance C C5H12 + O2 → 5 CO2 + H2O Balance H C5H12 + O2 → 5 CO2 + 6 H2O Balance O with O2 C5H12 + 8 O2 → 5 CO2 + 6 H2O balanced Halogenation of Alkanes (Substitution) When alkanes react with halogens,

The reaction is called substitution, one or more H atoms are replaced with a halogen usually Cl or Br. Light or heat is required. A mixture of halogenated products result.

CH4 + Cl2 → CH3Cl + HCl chloromethane

Cl2 CH2Cl2 + HCl dichloromethane

Cl2 CHCl3 + HCl trichloromethane (chloroform)

Cl2 CHCl4 + HCl tetrachloromethane, carbon tetrachloride

8 Reyes

Cyclopropane Cyclobutane Cyclopentane Cyclohexane Cycloheptane Cyclooctane

H2C CH2

CH2

Cyclopropane

H2C CH2

CH2H2C

Cyclobutane

H2C CH2

CH2

H2C

H2C

Cyclopentane Cyclohexane

H2C

H2C

CH2

CH2

CH2

H2C

(mp -128oC, bp -33oC) (mp -50oC, bp -12oC) (all bond angles

are near 109o)

(all bond angles

are near 109.5o)

CH3

methylcyclohexane

CH2CH3

1-ethyl-3-methylcyclohexane

CH3 12

3

45

6

12.9 Cycloalkanes Cycloalkanes

Are cyclic alkanes. Have 2H 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.

12.10 Drawing and Naming Cycloalkanes Line-bond (Geometrical) Formulas Cycloalkanes can be represented by polygons. Naming Cycloalkanes

STEP 1: Use the cycloalkane name as the parent. That is, compounds are named as alkyl-substituted cycloalkanes rather than as cycloalkyl-substituted alkanes. If there is only one substituent on the ring, it is not even necessary to assign a number because all ring positions are identical.

STEP 2: Identify and number the substituents. Start numbering at the group that has alphabetical priority, and proceed around the ring in the direction that gives the second substituent the lower possible number.