10.1 Fundamentals of Organic Chemistry

10.1.2 Homologous Series

• Carbon has the ability to form covalent bonds with itself, producing long chains and rings; this is known as catenation.

• Homologous series: is a series of closely-related compounds in organic chemistry.

• Alkanes are hydrocarbons — compounds composed of carbon and hydrogen atoms only.

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• Members of the same homologous series also have the same general formula and functional group.

• For the alkanes, the general formula is C_nH_2n+2

  • This allows us to determine the molecular formula of any alkane given the number of carbon atoms.

• The functional group of the alkanes is the alkyl group, which contains carbon-carbon single bonds.

Exam Tip

Make sure that you are able to explain the trend in the boiling points of the members of a homologous series. The boiling points increase with the number of carbon atoms in the molecule due to the increase in molar mass and stronger intermolecular forces.

• Intermolecular Forces: is the force that mediates interaction between molecules.

• In summary, a homologous series is a group of organic compounds that:

  • contain the same functional group,

  • have the same general formula,

  • differ by a CH₂ group

  • have similar chemical properties

  • show a gradation in physical properties such as boiling points

10.1.3 Molecular and Structural Formulae

• Organic compounds can be represented by three main different types of formula: empirical formulae, molecular formulae, and structural formulae.

• Structural formulae can be divided into full structural formulae, condensed structural formulae, and skeletal formulae.

Empirical and Molecular Formulae

• For some alkanes, methane, and propane, for example the empirical formula is the same as the molecular formula.

  • Though for compounds, this is not that case.

  • ex. the e.f. of ethane (C₂H₆) is CH₃.

• The molecular formula represents the actual number of each type of atom in a compound.

Structural Formulae

• In structural formulae, all atoms and the bonds between the atoms are shown.

• In full structural formulae, a single line represents a single covalent bond, a double line a double covalent bond and a triple line a triple covalent bond.

• Sometimes it is preferable to use condensed structural formulae, in which the bonds between atoms are omitted.

• In skeletal formulae, all atoms are omitted leaving only the backbone of the molecule.

• In this type of formula, carbon atoms are assumed to be at the intersection of each line and at the end of each line.

  • In addition, hydrogen atoms bonded to carbon atoms are not shown.

  • If hydrogen is part of a functional group (such as hydroxyl group, OH) it is shown.

Represented Formulae in 3-D

• When representing organic molecules in two-dimensions, we use structural formulae with 90 degree and 180 bond angles.

• Though this does not represent the actual molecular geometry of the molecule.

• When carbon forms for single bonds, the molecular geometry is tetrahedral with the bond at 109.5 degree to each other.

• When it for forms a double bond, as in ethene, the arrangement is trigonal planar with a bond angle of 120 degree.

  • For certain molecules, it is useful to show the relative three-dimensional positions of atoms or groups around a selected carbon atom, known as the stereochemical formula.

• In a stereochemical formula, a bond sticking out from the plane of the page is shown as a solid wedge, whereas a bond directed behind the page is shown as a broken line.

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Branched and Cyclic Structures

• Carbon atoms can also be joined together in rings, forming cyclic alkanes.

• The structures of these cyclic hydrocarbon molecules are often represented by polygons.

• The corners of the polygon represent a carbon atom together with the hydrogen atoms that are bonded to the carbon.

10.1.4 Structural Isomerism

• Structural Isomers: they are compounds with the same molecular formula but different arrangements of atoms.

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