Hydrocarbons

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Hydrocarbons

Functional Groups and Homologous Series

• Functional group: atom or group of atoms responsible for the typical chemical reactions of a molecule
• Homologous series is a group of molecules with the same functional group but a different number of -CH2 groups
• Family that differs by number of carbon atoms
• Functional groups determine the pattern of reactivity of a homologous series, whereas the carbon chain length determines physical properties such as melting/boiling points

Carbon

• Bonding capacity: 4
• Can form 4 covalent bonds
• 4 valence electrons
• Sharing 1 electron forms a covalent bond
• Carbon can make double or triple covalent bonds

Saturated Hydrocarbons

• Hydrocarbons with only single bonds between carbon atoms
• Simplest class of hydrocarbons
• Each carbon atom is bonded to as many hydrogen atoms as possible
• Makes them relatively unreactive/un-volatile, apart from their interaction with the oxygen in the air in the case of combustion

Structural Isomers

• Isomers: organic molecules with the same molecular formula but a different structural formula
• Structural isomers have:
  • Different names
  • Different shapes
  • Different chemical properties
• Read from left to right for isomers

Alkanes

• Family of hydrocarbons
• Saturated hydrocarbons with only single bonds between the carbon atoms
• Can be arranged in continuous straight chains or in branches or rings
• Hydrogen = 2 x carbon + 2

Naming Alkanes: Branched

1. Find the parent chain = longest continuous chain of carbon atoms in the molecule
2. Number the carbon atoms in the parent chain. Start at the end that will give the smallest numbers possible to the carbon atoms where the branches originate
3. Substituents = atoms attached to the parent chain. Alkyl group: A substituent that is a hydrocarbon. The names of alkyl groups use the same prefixes as the alkanes, but with a -yl suffix, e.g. methyl, ethyl, propyl, etc. Substituents are named by placing the carbon number from the parent carbon chain in front of the name with the substituent. 
4. Use a prefix to indicate the appearance of more than one of the same substituent in the structural formula. 2 of the same group is di-, 3 is tri-, 4 is tetra
5. Multiple different substituents are listed in alphabetical order. Ignore any of the prefixes from rule 4.
6. Commas are used to separate multiple numbers. Hyphens come between the number and name of a substituent. The parent name comes immediately after the last substituent. There are no spaces in the name

Melting and boiling points: Alkanes

• Alkanes have low melting or boiling points
• Due to very weak intermolecular forces between alkane molecules
• Methane, ethane and butane are gases at room temperature
• As the carbon chain gets longer, because the attraction between the molecules increases, therefore more energy is needed to overcome the attraction 
• Branched isomers have lower boiling/melting points, because the branches prevent the molecules from getting close so they cannot attract to each other as strongly

Van der Waal’s forces

• Named after Dutch scientist J D Van der Waal who discovered them
• The forces are created by the constant movements of electrons in atoms of molecules at high speeds
• At any instant in time there is the likelihood that one side of the molecule has a greater proportion of electrons
• This is called a dipole, and causes the molecule to have positively and negatively charged ends
• A temporary dipole or imbalance of charge can attract the electron cloud from a neighbouring atom or molecule, e.g. slightly positive end attracts negatively charged electrons
• The temporary dipole induces an imbalance of charges in neighbouring molecules
• The strength of the VdW force is dependent on the size of the molecules or atoms
• As the size and molecular mass of the molecule increases so does the number of electrons, resulting in a greater imbalance of charge, and therefore a greater VdW attraction
• This is why boiling points increase as the number of carbon atoms increase in alkanes, more energy is required to overcome the VdW forces

Viscosity of Alkanes

• Methane, ethane, propane, butane = gases at room temperature
• Alkanes from pentane through to C16 are liquids
• The viscosity of liquid alkanes increases as the number of carbon atoms increase
• The increase in viscosity is due to the increase of intermolecular forces between molecules of increasing size

Volatility of Alkanes

• High volatility: low boiling point
• High volatility: weak intermolecular forces, thus fewer number of carbons in straight chain the higher the volatility

Chemical Properties of Alkanes

• Combustion
  • Alkanes are flammable
  • Complete combustion (lots of oxygen present) produces carbon dioxide and water
  • Incomplete combustion (limited oxygen present) produces carbon monoxide and water

Alkenes

• Alkenes make up a family of hydrocarbons that each contain one double bond between 2 carbon atoms
• Unsaturated hydrocarbons
• C=C double bond
• The double bond is the functional group of the alkene homologous series
• Hydrogen = 2*carbon

Naming Alkenes: Straight Chain 

• Alkenes with carbon atoms arranged in a continuous straight line
• Retain the prefixes Eth, Prop, But, Pent, Hex, Hept, Oct for the parent compound
• Parent chain is the long chain containing the C=C
• Number the chain from the end that gives the C=C the lowest number
• Family suffix name: ene

Naming Alkenes: Structural Isomerism

• Alkenes containing 4+ carbons in the hydrocarbon chain, structural isomerism occurs
• e.g. C4 alkene C4H8 has 2 isomers
• Use the lower number of C where the double bond occurs for suffix n-ene
• The prefixes are still the same for alkanes

1. Find the parent chain = longest continuous chain of carbon atoms in the molecule with the C=C, this is given the stem name n-ene
2. Number the carbon atoms in the parent chain that will give the C=C the lowest number
3. Name the side chains in the same way as with alkanes

Geometrical Structural Isomers

• There is no rotation possible around a double bond
• 2 molecules can form where the position of the double bond remains the same but the spatial arrangement across the double bond is different
• Cis isomer: CH3 groups are on the same side of the double bond
• Trans isomer: CH3 groups are on opposite sides of the double bond

Properties of Alkenes

• Alkenes are more reactive than alkanes due to the relatively instability of the double bond
• Breaking a carbon-carbon bond from a double bond between carbon atoms requires less energy than breaking a C-C bond
• The boiling and melting points of alkenes are generally slightly lower than alkanes with the same amounts of carbon atoms.
• The boiling point of straight-chain alkenes is more than branched-chain alkenes just as in alkanes
• Alkenes with 2-4 carbon atoms = gases at room temperature
• Alkenes with 5-17 carbon atoms = liquids at room temperature
• Viscosity of alkenes increases with increased carbon chain length
• Volatility of alkenes decreases with increased carbon chain length

Alkynes

• Alkynes make up a family of hydrocarbons that each contain one triple bond between 2 carbon atoms
• Unsaturated hydrocarbons
• C☰C triple bond
• The triple bond is the functional group of the alkyne homologous series’
• Hydrogen = 2*carbon - 2

Naming Alkynes: Straight chain

• Alkynes with carbon atoms arranged in a continuous straight line
• Retain the prefixes Eth, Prop, But, Pent, Hex, Hept, Oct for the parent compound
• Parent compound is the longest chain containing the C☰C
• Number the chain from the end that gives the C☰C the lowest number
• Family name suffix: yne

Properties of Alkynes

• Ethyne, propyne and butyne = gases, C5-C12 = liquid
• Boiling points
  • Compared to alkanes and alkenes, alkynes have a slightly higher boiling point
• Viscosity of alkynes increases with increased carbon chain length
• Volatility of alkynes decreases with increased carbon chain length

Cycloalkanes

• Rings of carbon atoms with distinguishing chemical properties = cycloalkanes
• Cycloalkanes only contain carbon-hydrogen bonds and carbon-carbon single bonds, but in cycloalkanes, the carbon atoms are joined in a ring
• The smallest cycloalkane is cyclopropane
• Hydrogen = 2*carbon

Naming Cycloalkanes

• Use the number of carbons in the ring to determine the prefix Prop, But, Pent, Hex, Hept, Oct
• Suffix = n-ane
• Place cyclo- in front

Cycloalkenes

• Hydrogen = 2*(carbon - number of double bonds)

Naming Cycloalkenes

• Use the number of carbons in the ring to determine the prefix Prop, But, Pent, Hex, Hept, Oct
• Suffix = ene 
• Place cyclo- in front

Alcohols

• Homologous group of carbon compounds containing the hydroxyl (-OH) group
• Methanol, ethanol, propanol, etc
• General formula Cn, H2n+1, OH

Isomers

• Isomers are molecules with the same molecular formula (i.e. same number and type of atoms) but in which the atoms are arranged in a different way
• There are 2 main categories of isomerism: structural isomerism and stereoisomerism

Structural Isomerisms

• Different structural formulae
• Chain isomerism
  • Carbon chain is arranged differently, e.g. different shape, L shape instead of chain, cross shape
• Positional isomerism
  • Functional group is attached to a different carbon atom, e.g. 3-chloropentane, 2-chloropentane
  • Also exists in alkenes with 4 or more carbon atoms
• Functional group isomerism
  • Have different functional groups, so are members of different homologous series

Stereoisomerism

• Have same structural formula

• 3d arrangement of atoms is different

• 2 types

• Cis-trans isomerism

• Optical isomerism

  Cis-trans isomerism

  • Molecules can rotate freely around C-C covalent bonds, but not around C=C double bonds
  • Leads to cis-trans isomerism, in which isomers differ in the arrangement of the groups attached to the carbons in the double bonds
  • These isomers cannot be super imposed on each other because the arrangement of the methyl groups is different
  • If an alkyl group or atom other than hydrogen is attached to each carbon then the atoms can be named either cis (on the same side) or trans (on the opposite side)

  Limitations of Cis-Trans isomerism

  • In more complex organic compounds, in which multiple hydrogens have been substituted by different groups, isomers cannot be defined using the cis-trans notation
  • Instead, a different system is used for these type of molecules: E-Z notation

E-Z Notation

• Used to identify stereoisomers that cannot be called cis or trans
• Isomers are identified as either E or Z depending on what “priority” is given to the groups attached to the carbon atoms in the double bond
• Priority of these groups is determined by a series of complex rules
• E represents the German word “entgegen”(opposite) and corresponds to trans isomers
• The highest priority groups are on the opposite side of the double bond
• Z represents the German word “zusammen”(together) and corresponds to cis isomers
• The highest priority groups are on the same side of the double bond
• Highest priority, in this case, refers to the highest atomic number of the atoms

Optical isomerism

• A molecule can exist as 2 isomers that are non-superimposable, mirror images of each other, just like a left hand and right hand
• Optical isomers have the same physical properties but they rotate polarised light in opposite directions