#2: Alkanes and Cycloalkanes

Question 1

Hydrocarbons

Answer 1

The simplest organic compounds. Contain only carbon and hydrogen.

Divided into two main classes: aliphatic and aromatic.

Question 2

Aliphatic Hydrocarbons

Answer 2

Aliphatic means fat. So these are hydrocarbons that are a part of fats and oils.

Three major groups, alkanes, alkenes, and alkynes.

Question 3

Aromatic Hydrocarbons

Answer 3

Irrespective of their own odor, were typically obtained by chemical treatment of pleasant-smelling plant extracts.

Question 4

Alkanes

Answer 4

Hydrocarbons in which all the bonds are single bonds.

General Formula: CnH2n+2

Question 5

Alkenes

Answer 5

Hydrocarbons which contain at least one carbon-carbon double bond.

Question 6

Alkynes

Answer 6

Hydrocarbons which contain at least one carbon-carbon triple bond.

Question 7

Carbon Atoms and Boiling Point

Answer 7

It is generally true that as the number of carbon atoms increases, so does the boiling point.

Question 8

Methane

Answer 8

IUPAC name for 1 Carbon atom.

Question 9

Ethane

Answer 9

IUPAC name for 2 Carbon atoms.

Question 10

Propane

Answer 10

IUPAC name for 3 Carbon atoms.

Question 11

Butane

Answer 11

IUPAC name for 4 Carbon atoms.

Question 12

Pentane

Answer 12

IUPAC name for 5 Carbon atoms.

Question 13

Hexane

Answer 13

IUPAC name for 6 Carbon atoms.

Question 14

Heptane

Answer 14

IUPAC name for 7 Carbon atoms.

Question 15

Octane

Answer 15

IUPAC name for 8 Carbon atoms.

Question 16

Nonane

Answer 16

IUPAC name for 9 Carbon atoms.

Question 17

Decane

Answer 17

IUPAC name for 10 Carbon atoms.

Question 18

IUPAC Nomenclature of Alkanes Rules

Answer 18

1) Find the longest chain (parent).
2) Identify the substitution and number the parent chain so that the substitution gets the lowest number.
3) Name of the substitution ends with -yl and that of the parent ends with -ane.

4) When the parent chain has more than one substituent:
- Choose numbering so that least number is received by substituents.
- List the substituents alphabetically.
- Use prefix such as di, tri, and tetra if more than one of the same substituent is present. Prefix names are ignored when alphabetizing.
- Italicized predix sec-, tert- are also ignored when alphabetizing except when compared with each other. For example sec- comes before tert-.
- Iso- is part of the name and is included in alphabetizing.

5) When both the substituents get equal number from two different direction choose the direction that gives lower number to the substituent that appears first in the name.
6) When there are two possible parent (equal chain length) choose the parent chain that has greater number of substituents as the parent.

Question 19

SP3 Hybrid Orbitals

Answer 19

In the 1930s Linus Pauling suggested that the electron configuration of a carbon bonded to other atoms need not to be the same as a free carbon atom. By mixing (hybridizing) the 2s, 2px, 2py, and 2pz orbitals, four new orbitals are obtained. These four new orbitals are called sp3 hybrid orbitals because they come from one s orbital and three p orbitals. Each sp3 hybrid orbital has 25% s character and 75% p character. Among their most important features are the following:

1) All four sp3 orbitals are of equal energy. Therefore, according to Hund’s rule the four valence electrons or carbon are distributed equally among them, making four half-filled orbitals available for bonding.
2) The axes of the sp3 orbitals point toward the corners of a tetrahedron. Therefore, sp3 hybridization of carbon is consistent with the tetrahedeal structure of methane. Each C-H bond is a sigma bond in which a half-filled 1s orbital of hydrogen overlaps with a half-filled sp3 orbital of carbon along a ling drawn between them.
3) Sigma Bonds involving sp3 hybrid orbitals of carbon are stronger than those involving unhybridized 2s or 2p orbitals. Each sp3 hybrid orbital has two lobes of unequal size, making the electron density greater on one side of the nucleus than the other. In a C-H sigma bond, it’s the larger lobe of a carbon sp3 orbital that overlaps with a hydrogen 1s orbital. This concentrates the electron density in the region between the two atoms.

Question 20

Unbranched Alkanes

Answer 20

Straight line, no branching. Named n-alkane.

Question 21

Branched Alkanes

Answer 21

Parent Alkanes - carbons connected in straight line.

Substituents - hang from parent a.

Question 22

Alkyl Group

Answer 22

Lacks one of the hydrogens of an alkane. A methyl group (-CH3) is an alkyl group derived from methane (CH4). Unbranched alkyl groups in which the point of attachment is at the end of the chain are named in the IUPAC nomenclature by replacing the -ane endings by -yl.

Alkyl groups are designated as primary, secondary, or tertiary according to the degree of substitution of the carbon at the potential point of attachment.

Question 23

Primary Carbon

Answer 23

Directly attached to one other carbon.

Question 24

Secondary Carbon

Answer 24

Directly attached to two other carbons.

Question 25

Tertiary Carbon

Answer 25

Directly attached to three other carbons.

Question 26

Quaternary Carbon

Answer 26

Directly attached to four other carbons.

Question 27

Cycloalkanes

Answer 27

Alkanes that contain a ring of three or more carbons. Characterized by CnH2n.

Named under the IUPAC system by adding the prefix cyclo- to the name of the unbranched alkane with the same number of carbons as the ring.

Substitutions are identified in the usual way. Their positions are specified by numbering the carbon atoms of the ring in the direction that gives the lowest number to the substituents at the first point of difference.

If alkyl chain is shorter or the same length as the cycloalkane, the cycloalkane is parent.

Question 28

Phases of Alkanes

Answer 28

1-4 Carbon atoms, alkane is gas at room temperautre.

5-17 Carbon atoms, liquid at room temperature.

18+ Carbon atoms, gas at room temperature.

Question 29

Van der Waals Forces

Answer 29

Attractive forces between neutral species (atoms or molecules, not ions). 3 Types:

1) dipole-dipole (including Hydrogen bonding)
2) dipole/indiced-dipole
3) induced-dipole/induced-dipole

Most alkanes have no measurable dipole movement, and therefore the only van der Waals force to be considered is the induced-dipole/induced-dipole attractive force.

Question 30

Heat of Combustion

Answer 30

Heat released on combustion (burning) of a substance.

Unbranched alkanes have slightly higher heats of combustion than their 2-methyl-branched isomers, but the most important factor is the number of carbons. The unbraned alkanes and the 2-methyl-branched alkanes constitute two separate homologous series in which there’s a regular increase of about 653 kJ/mol in the heat of combustion for each additional CH2 group.

The higher the heat of Combustion is, the lower the stability or higher the reactivity of the alkane. So unbranched alkanes are less stable than branched alkanes.

Longer chain alkanes are also less stable than short-chain alkanes.

Question 31

Oxidation-Reduction in Organic Chemistry

Answer 31

Oxidation of carbon corresponds to an increase in the number of bonds between carbon and oxygen or to a decrease in the number of carbon-hydrogen bonds.

Conversely, reducing corresponds to an increase in the number of carbon-hydrogen bonds or to a decrease in the number of carbon-oxygen bonds.

Question 32

Oxidation-Reduction in terms of Electronegativity

Answer 32

Oxidation of carbon occurs when a bond between a carbon and an atom that’s less electronegative than carbon is replaced by a bond to an atom that is more electronegative than carbon. Reverse process is reduction.

Oxidation goes from less to more electronegative. Reduction is reverse.