Displayed Formula of Molecules

• The displayed formula represents all atoms of each element and all bonds in a molecule.

• Example: The molecular formula of methane is $CH_4$.

  • The displayed formula for methane illustrates the structure:

  H
  |
  H-C-H
  |
  H

• Each hydrogen atom can form 1 bond, whereas one carbon atom can form 4 bonds.

• Example: The molecular formula for ethanoic acid is $CH_3COOH$.

  • The displayed formula of ethanoic acid is:

  H O
  | ||
  H-C-C-O-H
  |
  H

• In this case, hydrogen atoms form 1 bond, carbon atoms can form 4 bonds, and oxygen atoms can form 2 bonds.

General Formula of Compounds in Homologous Series

• The general formula refers to the formula applicable to all the compounds in a homologous series, representative of the composition of atoms in each molecule.

(a) Alkanes (CₙH₂ₙ₊₂)

• The general formula for an alkane is: $C_nH_{2n+2}$.

• Example: Calculate the molecular formula of an alkane with 5 carbon atoms.

  • Let $n = 5$.

  • Calculate the number of hydrogen atoms: $2(5) + 2 = 12$.

  • The molecular formula is $C_5H_{12}$ (pentane).

(b) Alkenes (CₙH₂ₙ)

• The general formula for an alkene is: $C_nH_{2n}$.

• Example: Calculate the molecular formula of an alkene with 4 carbon atoms.

  • Let $n = 4$.

  • Calculate the number of hydrogen atoms: $2(4) = 8$.

  • The molecular formula is $C_4H_8$ (butene).

(c) Alcohols (CₙH₂ₙ₊₁OH)

• The general formula for an alcohol is: $C_nH_{2n+1}OH$.

• Example: Calculate the molecular formula of an alcohol with 4 carbon atoms.

  • Let $n = 4$.

  • Calculate the number of hydrogen atoms: $2(4) + 1 = 9$.

  • The molecular formula is $C_4H_9OH$ (butanol).

(d) Carboxylic Acids (CₙH₂ₙ₊₁COOH)

• The general formula for a carboxylic acid is: $C_nH_{2n+1}COOH$.

• Example: Calculate the formula of a carboxylic acid with 4 carbon atoms.

  • Total carbon atoms including the functional group $COOH$; let $n = 3$.

  • Calculate the number of hydrogen atoms: $2(3) + 1 = 7$.

  • The formula is $C_3H_7COOH$ (butanoic acid).

Identify a Functional Group

• A functional group comprises an atom or group of atoms that determine the chemical properties of a homologous series.

  • Example: The functional group of an alcohol is the hydroxyl group (-OH).

Characteristics of a Homologous Series

• A homologous series is defined as a family or group of similar compounds sharing similar chemical properties due to the presence of the same functional group.

Saturated vs. Unsaturated Compounds

Saturated Compounds

• A saturated compound has molecules where all carbon-carbon bonds are single bonds.

  • Example: All alkanes are saturated since they only have single carbon-carbon bonds.

Unsaturated Compounds

• An unsaturated compound has molecules that contain one or more carbon-carbon bonds that are not single.

  • Example: Alkenes are unsaturated because they contain at least one double carbon-carbon bond.

Structural Formula

• A structural formula is an unambiguous description of the arrangement of atoms in a molecule, without necessarily indicating all bonds.

  • Example: The structural formula for ethene is $CH_2=CH_2$ (molecular formula: $C_2H_4$).

  • Example: The structural formula for ethanol is $CH_3CH_2OH$ (molecular formula: $C_2H_5OH$).

  • Example: The structural formula for methyl ethanoate is $CH_3COOCH_3$ (molecular formula: $C_3H_6O_2$).

Structural Isomers

• Structural isomers are compounds that share the same molecular formula but have different structural formulae.

  • Example: C₄H₁₀ has structural isomers:

  • Butane as $CH_3CH_2CH_2CH_3$.

  • 2-methylpropane as $CH_3CH(CH_3)CH_2CH_3$.

  • The difference arises from having a side group (methyl group) in 2-methylpropane versus a straight chain in butane.

  • Example: C₄H₈ includes structural isomers such as:

  • $CH_3CH_2CH=CH_2$ representing 1-butene.

  • $CH_3CH=CHCH_3$ representing 2-butene.

  • The differences are in the position of the double bond.

General Characteristics of a Homologous Series (Extended Only)

• The general characteristics of a homologous series are as follows:

  • Members have the same functional group.

  • All members share the same general formula.

  • The difference between one member and the next is a –CH₂– unit.

  • Members display a trend in physical properties (e.g. boiling points, melting points).

  • Members exhibit similar chemical properties due to the same functional group.

The displayed formula represents all atoms of each element and all bonds in a molecule. For instance, the molecular formula of methane is $CH_4$, and its displayed formula illustrates the structure: H | H-C-H | H. Each hydrogen atom can form 1 bond, while one carbon atom can form 4 bonds. Another example is the molecular formula for ethanoic acid, which is $CH_3COOH$, and its displayed formula is: H O | || H-C-C-O-H | H. In this case, hydrogen atoms form 1 bond, carbon atoms can form 4 bonds, and oxygen atoms can form 2 bonds.

The general formula refers to the formula applicable to all the compounds in a homologous series, representing the composition of atoms in each molecule. For alkanes, the general formula is $C_nH_{2n+2}$. To illustrate, if we calculate the molecular formula of an alkane with 5 carbon atoms (letting $n = 5$), the number of hydrogen atoms can be calculated as $2(5) + 2 = 12$, leading to the molecular formula of $C_5H_{12}$ (pentane). For alkenes, the general formula is $C_nH_{2n}$. Again, by calculating the molecular formula for an alkene with 4 carbon atoms (letting $n = 4$), the number of hydrogen atoms is $2(4) = 8$, resulting in the molecular formula of $C_4H_8$ (butene). Alcohols have the general formula $C_nH_{2n+1}OH$; for 4 carbon atoms, the calculation yields $2(4) + 1 = 9$, leading to the formula $C_4H_9OH$ (butanol). Carboxylic acids follow the general formula $C_nH_{2n+1}COOH$, and for a carboxylic acid with 4 carbon atoms, we use $n = 3$ to account for the carbon atoms in the functional group $COOH$. The number of hydrogen atoms is calculated as $2(3) + 1 = 7$ leading to the formula $C_3H_7COOH$ (butanoic acid).

A functional group comprises an atom or group of atoms that determine the chemical properties of a homologous series; for example, the functional group of an alcohol is the hydroxyl group (-OH). A homologous series is defined as a family or group of similar compounds that share similar chemical properties due to the presence of the same functional group.

Saturated compounds are characterized by having molecules where all carbon-carbon bonds are single bonds; all alkanes are considered saturated. On the other hand, unsaturated compounds have molecules with one or more carbon-carbon bonds that are not single. Alkenes serve as an example of unsaturated compounds since they contain at least one double carbon-carbon bond.

A structural formula provides an unambiguous description of the arrangement of atoms in a molecule, without necessarily showing all the bonds. For example, the structural formula for ethene can be given as $CH_2=CH_2$ (molecular formula: $C_2H_4$), while the structural formula for ethanol is $CH_3CH_2OH$ (molecular formula: $C_2H_5OH$), and for methyl ethanoate, it is $CH_3COOCH_3$ (molecular formula: $C_3H_6O_2$).

Structural isomers are compounds sharing the same molecular formula but have different structural formulae. For instance, $C₄H₁₀$ has structural isomers such as butane, represented as $CH_3CH_2CH_2CH_3$, and 2-methylpropane, which is $CH_3CH(CH_3)CH_2CH_3$. The difference results from having a side group (methyl group) in 2-methylpropane, in contrast to a straight chain in butane. Another example, $C₄H₈$, includes structural isomers such as $CH_3CH_2CH=CH_2$ (1-butene) and $CH_3CH=CHCH_3$ (2-butene), with differences arising from the position of the double bond.

The general characteristics of a homologous series include members having the same functional group, sharing the same general formula, and differing from one member to the next by a –CH₂– unit. These members also display a trend in physical properties, such as boiling points and melting points, and exhibit similar chemical properties due to the presence of the same functional group.