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).

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• Compounds that have the same molecular formula but different structural formulas are called isomers of one another.

• Compounds with all their carbons connected in a continuous chain are called straight-chain alkanes; those with a branching connection of carbons are called branched-chain alkanes.

• Constitutional isomers are compounds with the same molecular formula but different connections among their atoms. Also known as structural isomers.

• Functional group isomer are isomers having the same chemical formula but belonging to different chemical families due to differences in bonding; ethanol and dimethyl ether are examples of functional group isomers.

• Line structure, also known as line-angle structure; a shorthand way of drawing structures in which carbon and hydrogen atoms are not explicitly shown. Instead, a carbon atom is understood to be wherever a line begins or ends and at every intersection of two lines, and hydrogens are understood to be wherever they are needed to have each carbon form four bonds.

• Conformation is the specific three-dimensional arrangement of atoms in a molecule achieved specifically through rotations around carbon–carbon single bonds.

• Conformer are the molecular structures having identical connections between atoms where the interconversion of C¬C bond rotations results only in a different spatial arrangement of atoms.

• Substituent is an atom or group of atoms attached to a parent compound.

• A primary 1° carbon atom has one other carbon attached to it (typically indicated as an —R group in the molecular structure), a secondary 2° carbon atom has two other carbons attached, a tertiary 3° carbon atom has three other carbons attached, and a quaternary 4° carbon atom has four other carbons attached.

• Branched-chain alkanes can be named by following four steps:

  • 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.

• There are three major intermolecular forces involving organic molecules: dipole–dipole forces, hydrogen bonding and London dispersion forces.

  • 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.

• Properties of Alkanes:

  • 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

• Alkanes do not react with acids, bases, or most other common laboratory reagents (a substance that causes a reaction to occur). Their only major reactions are with oxygen (combustion) and with halogens (halogenation). Both of these reaction types have complicated mechanisms and occur through the intermediacy of free radicals.

  • 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).

• Cycloalkane is an alkane that contains a ring of carbon atoms.

  • 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.^^

• Cycloalkanes are named by a straightforward extension of the rules for naming open-chain alkanes.

  • 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.

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