Chapter 12:Introduction to organic chemistry: Alkanes

^^Carbon is tetravalent^^; it always forms four bonds. With the four valence electrons it already possesses, carbon has the ability to pick up four more from other atoms to fill out its octet.

In the organic compound methane, for example, carbon is connected to four hydrogen atoms, with each hydrogen donating its valence electron to carbon to fill out its octet. Because it has groups attached to the carbon, methane is both tetrahedral and tetravalent.

Some basic conclusions:

1. A carbon that has four groups attached will be ^^tetrahedral^^ (e.g., methane or ethane);
2. A carbon that has three groups attached will be ^^trigonal planar^^ (e.g., ethene);
3. A carbon that has two groups attached will be ^^linear^^ (e.g., ethyne).

A functional group is an atom or group of atoms that has a characteristic physical and chemical behaviour. Each functional group is always part of a larger molecule, and a molecule may have more than one class of functional group present.

• An important property of functional groups is that a given functional group tends to undergo the same types of reactions in every molecule that contains it.

The chemistry of an organic molecule is primarily determined by the functional groups it contains, not by its size or complexity.

The basic organic families are:

• ^^Hydrocarbons^^ are organic compounds that contain only carbon and hydrogen. Alkanes have only single bonds and contain no functional groups. The absence of functional groups makes alkanes relatively unreactive.
• Alkenes contain a carbon–carbon double-bond functional group; alkynes contain a carbon–carbon triple-bond functional group; and aromatic compounds contain a six-membered benzene ring of carbon atoms with three alternating double bonds.
• ^^Alkyl halides^^ have a carbon–halogen bond; ^^alcohols^^ have a carbon–oxygen bond; ^^ethers^^ have two carbons bonded to the same oxygen; and ^^amines^^ have a carbon–nitrogen bond.
• Functional groups that contain a carbon– oxygen double bond: ^^aldehydes, ketones, carboxylic acids, anhydrides, esters, and amides.^^
• Functional groups that contain ^^sulfur: thioalcohols (known simply as thiols), sulfides, and disulfides.^^ These three families play an important role in protein function.
• Many of the organic molecules will have more than one functional group present in the same molecule .
• When this is the case, we will classify the molecule as chemically belonging to multiple functional group families; from a biological and medical standpoint these molecules are quite often classified according their biologically relevant function (e.g., neurotransmitters or nucleic acids).

• STEP 1: Name the main chain. Find the longest continuous chain of carbons, and name the chain according to the number of carbon atoms it contains.
• STEP 2: Number the carbon atoms in the main chain, beginning at the end nearer the first branch point.
• STEP 3: Identify the branching substituents, and number each according to its point of attachment to the main chain.
• STEP 4: Write the name as a single word, using hyphens to separate the numbers from the different prefixes and commas to separate numbers, if necessary.
• If two or more different substituent groups are present, cite them in alphabetical order.
• If two or more identical substituents are present, use one of the prefixes di-, tri-, tetra-, and so forth, but do not use these prefixes for alphabetizing purposes.

• Intermolecular forces are what cause molecules to aggregate or “stick” to one another; hydrogen bonds are the strongest, dipole–dipole forces follow in strength, and London dispersion forces are the weakest.

• Odourless or mild odour; colourless; tasteless; nontoxic
• Nonpolar; insoluble in water but soluble in nonpolar organic solvents; less dense than water
• Flammable; otherwise not very reactive

• Combustion is a chemical reaction that produces a flame, usually because of burning with oxygen.
• Halogenation is the replacement of an alkane hydrogen by a chlorine or bromine in a process initiated by heat or light. This process is known as “free radical halogenation” and occurs in a step-wise manner (a “free radical”, or a “radical”, is a molecule or atom containing a single, unpaired electron; since a radical does not have an octet of electrons around all of its atoms, it is highly reactive).

• Because of their cyclic structures, cycloalkane molecules are more rigid and less flexible than their open-chain counterparts.
• Rotation is not possible around the carbon–carbon bonds in cycloalkanes without breaking open the ring.
• ^^This property is known as restricted rotation and can lead to isomer formation.^^

• 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 lowest possible number.