Number of Constitutionally Isomeric Alkanes CH 4 1 C 8 H C 2 H 6 1 C 9 H C 3 H 8 1 C 10 H C 4 H 10 2 C 15 H 32 4,347 C 5 H 12 3 C 20 H ,319 C 6 H 14 5 C 40 H 82 62,491,178,805,831 C 7 H 16 9
Nomenclature of Alkanes Rules for naming compounds are given by the International Union for Pure and Applied Chemistry (IUPAC). To name alkanes: Find the longest chain and use it as the name of the compound. Number the carbon atoms starting with the end closest to the substituent. Name and give the location of each substituent. When two or more substituents are present list them in alphabetical order.
Nomenclature of Alkanes 1. Determine the number of carbons in the parent hydrocarbon 2. Number the chain so that the substituent gets the lowest possible number
3. Number the substituents to yield the lowest possible number in the number of the compound (substituents are listed in alphabetical order) 4. Assign the lowest possible numbers to all of the substituents
5. When both directions lead to the same lowest number for one of the substituents, the direction is chosen that gives the lowest possible number to one of the remaining substituents 6. If the same number is obtained in both directions, the first group receives the lowest number
7. In the case of two hydrocarbon chains with the same number of carbons, choose the one with the most substituents 8. Certain common nomenclatures are used in the IUPAC system
Nomenclature of Alkanes
Types of Carbons primary (1 o ) - bonded to only 1 other carbon secondary (2 o ) - bonded to 2 other carbons tertiary (3 o ) - bonded to 3 other carbons quaternary (4 o ) - bonded to 4 other carbons
Conformations are different spatial arrangements of a molecule that are generated by rotation about single bonds.
Eclipsed conformation Ethane
Staggered conformation Ethane
Projection formulas of the staggered conformation of ethane NewmanSawhorseHH HH HH HH H H H H
Anti relationshipsHH HH HH HH H H H H Two bonds are anti when the angle between them is 180°. 180°
Gauche relationships H H HH HH H H H H H H Two bonds are gauche when the angle between them is 60°. 60°
An important point: The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations.
0° 60° 120° 180° 240°300°360° 12 kJ/mol
Conformational Analysis of Butane: C 2 -C 3 Rotation
The most stable conformation of unbranched alkanes has anti relationships between carbons Hexane
Reactions with Alkanes The C-C and C-H bonds are very strong. Therefore, alkanes are very unreactive. At room temperature alkanes do not react with acids, bases, or strong oxidizing agents. Alkanes do combust in air (making them good fuels): 2C 2 H 6 (g) + 7O 2 (g) 4CO 2 (g) + 6H 2 O(l) H = kJ
Alkanes are very unreactive compounds because they have only strong s bonds and atoms with no partial charges However, alkanes do react with Cl 2 and Br 2
Reaction of Alkane with Cl 2 or Br 2
Petroleum is a complex mixture of alkanes and cycloalkanes that can be separated by distillation