McMurry Chapter 3 Organic Compounds: Alkanes and Their Stereochemistry Organic Chemistry I S. Imbriglio

Functional Groups A functional group is a group of atoms within a molecule that has a ___________________________. The chemistry of every organic molecule, regardless of size and complexity, is determined by the ______________________________ it contains.

Functional Groups Molecules are classified by the functional group(s) they contain. Akenes: contain carbon-carbon ________________ bonds Alkynes: contain carbon-carbon ______________ bonds Arenes: contain alternating double and single bonds in a six- membered ring of carbon atoms

Functional Groups These polar C-X bonds each exhibit unique but similar reactivity X = Cl, Br, I, O, N, S

Functional Groups All ________ ____________ contain a highly polarized C=O double bond and exhibit similar reactivity

Alkanes Alkanes , also called aliphatic compounds, are defined as molecules that ______________________________. Alkanes are also referred to as saturated hydrocarbons. Saturated – containing only single bonds; no pi bonds or rings Hydrocarbon – made up of only carbon and hydrogen

Alkanes All saturated hydrocarbons have the molecular formula CnH2n+2 Alkanes with carbons connected in a row are called _________________________________ alkanes. Straight-chain alkanes are named based on the number of carbon atoms they contain: n = 1 CH4 methane n = 7 C7H16 heptane n = 2 C2H6 ethane n = 8 C8H18 octane n = 3 C3H8 propane n = 9 C9H20 nonane n = 4 C4H10 butane n = 10 C10H22 decane n = 5 C5H12 pentane n = 11 C11H24 undecane n = 6 C6H14 hexane n = 12 C12H26 dodecane

Alkanes When there are more than three carbons in an alkane, it is possible to have normal and branched-chain isomers. ________________________: molecules that have the same molecular formulas but the atoms are connected in a different order

Constitutional Isomers There are two different constitutional isomers of butane and three different constitutional isomers of pentane.

Draw all possible constitutional isomers of hexane. How many are there?

Alkanes Alkyl Groups When an ____________________________ is part of a larger compound, it is referred to as an alkyl group. Alkyl groups are named by removing the –ane suffix from the parent alkane and replacing it with –yl.

Alkyl Groups There are two different propyl groups because there are two different types of ______________ hydrogens that can be removed from propane.

How many different butyl groups can you draw (each with the formula C4H9)?

Alkyl Groups _____________________________________________ One way to distinguish different alkyl groups is by the number of other carbon atoms attached to the branching carbon.

Primary (1), Secondary (2), and Tertiary (3) Used to classify molecules and distinguish between different sites within a molecule

Butyl Groups – Common Names Alkyl Groups 1°, 2°, and 3° are used to differentiate between the four butyl isomers. But there are eight different pentyl groups and seventeen hexyl groups. We need a more systematic way of naming alkanes and alkyl groups. Butyl Groups – Common Names

IUPAC Nomenclature The International Union of Pure and Applied Chemistry (IUPAC) developed a systematic way of naming organic molecules. Every IUPAC name uses the parent alkane backbone as the base name.

IUPAC Nomenclature We’ll use this molecule as an example as we walk through the naming rules…

IUPAC Nomenclature Step 1: Find the longest carbon chain. If there are two different chains of equal length, choose the one with the larger number of branch points as the parent chain. Step 2: Number the carbon chain, beginning with the end closest to the nearest branch point. If there is branching an equal distance away from both ends, begin numbering at the end nearer the second branch point. The first point of difference breaks a tie.

IUPAC Nomenclature Step 3: Identify and number the substituents. Use prefixes (di-, tri-, tetra-, etc.) if there is more than one of the same kind of substituent on the chain. Step 4: Write the name as a single word, listing the substituents alphabetically before the parent alkane. Use commas to separate numbers and hyphens to separate numbers from words. Do not alphabetize multiplier prefixes (di, tri, tetra, etc.).

IUPAC Nomenclature Complex Substituents Use IUPAC rules to name branched substituents. Begin numbering the longest carbon chain at the point of attachment. Alphabetize complex substituent by the first letter of the complete name (including numerical prefixes). Put name of alkyl group in parentheses.

IUPAC Nomenclature Common Names Some branched-chain alkyl groups are often named using their common names. You should be familiar with these names and structures. -Iso and neo are the only prefixes that are not hyphenated and are used when alphabetizing.

Provide IUPAC names for the following compounds.

Provide structures for the following IUPAC names. 4-t-butylheptane 3,4,5-trimethyl-4-n-propyloctane

Properties of Alkanes Alkanes are fairly inert, but they will undergo _________________________________________. Combustion: CH4 + 2 O2  CO2 + 2 H2O H = -890 kJ/mol Halogenation:

Properties of Alkanes Alkanes are __________ and tend to be ___________ in and __________ with polar solvents. The melting and boiling points of alkanes increase with increasing size – due to increased London dispersion forces.

Conformational Analysis of Alkanes _____________________: study of the three-dimensional arrangement of atoms in molecules _____________________: study of the energetics of different molecular conformations Conformations: structures related by bond rotations; usually interconvertible at room temperature Conformer: short for conformational isomer; term used to indicate a specific conformation

Conformational Analysis of Alkanes There is free rotation around C-C sigma bonds. Different conformations have different energies (different levels of stability).

Conformational Analysis of Alkanes _________________________ Newman projections are conformational drawings that allow you to look straight down a bond. Very useful for comparing conformers

Conformational Analysis of Ethane Staggered  = 60  Erel = 0 kJ/mol Erel = 0 kcal/mol Skewed 0 <  < 60  0 < Erel < 12 kJ/mol 0 < Erel < 3.0 kcal/mol Eclipsed  = 0  Erel = ~12 kJ/mol Erel = 3.0 kcal/mol

Draw Newman projections for the eclipsed and staggered conformers of ethane.

Rotational Energy Diagram for Ethane (3.0 kcal/mol) Barrier to Rotation = Ehighest – Elowest Ethane: Barrier = 3.0 – 0 = 3.0 kcal/mol

Conformational Analysis of Ethane The eclipsed conformer of ethane is higher in energy than the staggered conformer because of torsional strain. _______________________________strain caused by electron repulsion between eclipsed bonds As the dihedral angle gets smaller, the bonds are forced closer together, and the repulsion is increased. The angle between the bonds () is at a maximum in in the low energy staggered conformer (60) and at a minimum in the high energy eclipsed conformer (0).

Conformational Analysis of Ethane The conformational energies of ethane can be broken down into individual eclipsing interactions. Three pairs of eclipsed hydrogen bonds lead to 3.0 kcal/mol of torsional strain. One H/H-eclipsing interaction = 1.0 kcal/mol

Draw a Newman projection of propane from the perspective indicated.

Conformational Analysis of Propane Staggered  = 60  Erel = 0 kJ/mol Erel = 0 kcal/mol Eclipsed  = 0  Erel = ~14 kJ/mol Erel = 3.4 kcal/mol

Conformational Analysis of Propane The conformational energies of propane can be broken down into individual eclipsing interactions. Two pairs of eclipsed hydrogen bonds lead to 2.0 kcal/mol of torsional strain. One H/CH3-eclipsing interaction = 1.4 kcal/mol

Draw a Newman projection of the C2-C3 bond of butane from the perspective shown.

There are two energetically different staggered conformations of butane with respect to the C2-C3 bond. Draw a Newman projection for each and predict which one is lower in energy.

There are two energetically different eclipsed conformations of butane with respect to the C2-C3 bond. Draw a Newman projection for each and predict which one is higher in energy.

Conformational Analysis of Butane To distinguish between the four different conformers of butane, Me/Me is defined as the dihedral angle between the two methyl groups.

Conformational Analysis of Butane Butane: Barrier = 4.5 – 0 = 4.5 kcal/mol