CH 3: Organic Compounds: Alkanes and Their Stereochemistry Renee Y. Becker CHM 2210 Valencia Community College

We will take an initial look at 3-D aspects of molecules Why this Chapter Alkanes are unreactive, but provide useful vehicle to introduce important ideas about organic compounds Alkanes will be used to discuss basic approaches to naming organic compounds We will take an initial look at 3-D aspects of molecules

Functional Groups Functional group - collection of atoms at a site that have a characteristic behavior in all molecules where it occurs The group reacts in a typical way, generally independent of the rest of the molecule For example, the double bonds in simple and complex alkenes react with bromine in the same way

Functional Groups with Multiple Carbon–Carbon Bonds Alkenes have a C-C double bond Alkynes have a C-C triple bond Arenes have special bonds that are represented as alternating single and double C-C bonds in a six-membered ring  &  bonds?

Functional Groups with Carbon Singly Bonded to an Electronegative Atom

Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups)

Connecting carbons can lead to large or small molecules Alkanes 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 They are also called aliphatic compounds

Naming Alkanes Memorize 1-10 for next class

Alkane Isomers The molecular formula of an alkane with more than three carbons can give more than one structure C4 (butane) = butane and isobutane C5 (pentane) = pentane, 2-methylbutane, and 2,2-dimethylpropane Alkanes with C’s connected to no more than 2 other C’s are straight-chain or normal alkanes Alkanes with one or more C’s connected to 3 or 4 C’s are branched-chain alkanes

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

Condensed Structures of Alkanes We can represent an alkane in a brief form or in many types of extended form A condensed structure does not show bonds but lists atoms, such as CH3CH2CH2CH3 (butane) CH3(CH2)2CH3 (butane)

Alkyl group – remove one H from an alkane (a part of a structure) Alkyl Groups Alkyl group – remove one H from an alkane (a part of a structure) General abbreviation “R” (for Radical, an incomplete species or the “rest” of the molecule) Name: replace -ane ending of alkane with -yl ending CH3 is “methyl” (from methane) CH2CH3 is “ethyl” from ethane

Classified by the connection site (See Figure 3.3) 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)

Types of Hydrogens

Substituents

Isobutane, “isomer of butane” Common Names Isobutane, “isomer of butane” Isopentane, isohexane, etc., methyl branch on next-to-last carbon in chain. Neopentane, most highly branched Five possible isomers of hexane, 18 isomers of octane and 75 for decane!

Pentanes

Find the longest continuous carbon chain. IUPAC Names Find the longest continuous carbon chain. Number the carbons, starting closest to the first branch. Name the groups attached to the chain, using the carbon number as the locator. Alphabetize substituents. Use di-, tri-, etc., for multiples of same substituent.

Longest Chain The number of carbons in the longest chain determines the base name: ethane, hexane. If there are two possible chains with the same number of carbons, use the chain with the most substituents.

Start at the end closest to the first attached group. Number the Carbons Start at the end closest to the first attached group. If two substituents are equidistant, look for the next closest group. 1 2 3 4 5 6 7

CH3-, methyl CH3CH2-, ethyl CH3CH2CH2-, n-propyl Name Alkyl Groups CH3-, methyl CH3CH2-, ethyl CH3CH2CH2-, n-propyl CH3CH2CH2CH2-, n-butyl

H H n-propyl isopropyl A primary carbon A secondary carbon Propyl Groups H H n-propyl isopropyl A primary carbon A secondary carbon

Butyl Groups H H n-butyl sec-butyl A primary carbon A secondary carbon

H H tert-butyl isobutyl A tertiary carbon A primary carbon Isobutyl Groups H H isobutyl tert-butyl A tertiary carbon A primary carbon

Alphabetize substituents by name. Ignore di-, tri-, etc. for alphabetizing. 3-ethyl-2,6-dimethylheptane

Write structures for the following: a) 3-ethyl-3-methylpentane Example 1 Write structures for the following: a) 3-ethyl-3-methylpentane b) 4-t-butyl-2-methylheptane c) 5-isopropyl-3,3,4-trimethyloctane d) 3-ethyl-2,4,5-trimethylheptane

Provide IUPAC & common names (1-3) Example 2 Provide IUPAC & common names (1-3)

Name the following 3,3-dimethyl-4-isobutylheptane Example 3 Name the following 3,3-dimethyl-4-isobutylheptane 3,3-dimethyl-4-tert-butylheptane 4-tert-butyl-3,3-dimethylheptane 4-isobutyl-3,3-dimethylheptane

Name the branch off the branch using a locator number. Complex Substituents If the branch has a branch, number the carbons from the point of attachment. Name the branch off the branch using a locator number. Parentheses are used around the complex branch name. For alphabetizing use the first letter of the complex sub. Even if it is a numerical (di, tri, etc)

Complex Examples

Draw the structures and give their more common names Example 4 Draw the structures and give their more common names a) (1-methylethyl) group b) (2-methylpropyl) group c) (1-methylpropyl) group d) (1,1-dimethylethyl) group

4-(1-methylethyl)heptane Example 5 Draw the structures 4-(1-methylethyl)heptane b) 5-(1,2,2-trimethylpropyl)nonane

Assignment For next class draw the 9 isomers of heptane and name them

They will burn in a flame, producing carbon dioxide, water, and heat Properties of Alkanes Called paraffins (low affinity compounds) because they do not react as most chemicals They will burn in a flame, producing carbon dioxide, water, and heat They react with Cl2 in the presence of light to replace H’s with Cl’s (not controlled)

Physical Properties: Alkanes Solubility: hydrophobic Density: less than 1 g/mL Boiling points increase with increasing carbons (little less for branched chains). Melting points increase with increasing carbons (less for odd-number of carbons).

Boiling Points of Alkanes Branched alkanes have less surface area contact, so weaker intermolecular forces.

Melting Points of Alkanes Branched alkanes pack more efficiently into a crystalline structure, so have higher m.p.

Lower b.p. with increased branching Branched Alkanes Lower b.p. with increased branching Higher m.p. with increased branching H C 3 2 bp 60°C mp -154°C bp 58°C mp -135°C bp 50°C mp -98°C

Example 6 List each set of compounds in order of increasing boiling point and melting point

Which of the following has the highest boiling point? Example 7 Which of the following has the highest boiling point? 3-methylpentane 2,2-dimethylbutane Hexane Methane

Conformations can be represented in 2 ways: Conformers Conformation- Different arrangement of atoms resulting from bond rotation Conformations can be represented in 2 ways:

We do not observe perfectly free rotation Torsional Strain We do not observe perfectly free rotation There is a barrier to rotation, and some conformers are more stable than others Staggered- most stable: all 6 C-H bonds are as far away as possible Eclipsed- least stable: all 6 C-H bonds are as close as possible to each other

Conformations of Ethane Stereochemistry concerned with the 3-D aspects of molecules Rotation is possible around C-C bonds in open-chain molecules (not cyclic)

Eclipsed conformer has highest energy Dihedral angle = 0 degrees Ethane Conformers Eclipsed conformer has highest energy Dihedral angle = 0 degrees

Conformational Analysis Torsional strain: resistance to rotation. For ethane, only 3.0 kcal/mol

Propane Conformers Note slight increase in torsional strain due to the more bulky methyl group.

Highest energy has methyl groups eclipsed. Steric hindrance Butane Conformers C2-C3 Highest energy has methyl groups eclipsed. Steric hindrance Dihedral angle = 0 degrees totally eclipsed

Lowest energy has methyl groups anti. Dihedral angle = 180 degrees Butane Conformers (2) Lowest energy has methyl groups anti. Dihedral angle = 180 degrees anti

Methyl groups eclipsed with hydrogens Butane Conformers (3) Methyl groups eclipsed with hydrogens Higher energy than staggered conformer Dihedral angle = 120 degrees eclipsed

Gauche, staggered conformer Methyls closer than in anti conformer Butane Conformers (4) Gauche, staggered conformer Methyls closer than in anti conformer Dihedral angle = 60 degrees gauche

Conformational Analysis

Anti conformation is lowest in energy. Higher Alkanes Anti conformation is lowest in energy. “Straight chain” actually is zigzag.