Hydrocarbons

 
This is part of HSC Chemistry course under the topic of Hydrocarbons

HSC Chemistry Syllabus

• construct models, identify the functional group, and write structural and molecular formulae for homologous series of organic chemical compounds, up to C8 (ACSCH035): 

• conduct an investigation to compare the properties of organic chemical compounds within a homologous series, and explain these differences in terms of bonding (ACSCH035)

• analyse the shape of molecules formed between carbon atoms when a single, double or triple bond is formed between them

• explain the properties within and between the homologous series of alkanes with reference to the intermolecular and intramolecular bonding present

What are Hydrocarbons?

This video introduces organic compounds which are carbon-based molecules. Hydrocarbons are the simplest form of organic substances. Hydrocarbons are made from carbon and hydrogen atoms only. 

What is Organic Chemistry?

Organic chemistry involves the study of organic substances. Organic substances are compounds that contain carbon atoms, except: 

• oxides of carbon e.g. carbon dioxide, carbon monoxide and
• some ions containing carbon e.g. cyanide (CN^–).

 

Carbon atoms allow a wide diversity of molecules to be formed because it can bond with up to 4 other atoms. Carbon forms the fundamental building block for many substances found in living organisms, such as:

• Protein (polypeptide)
• Deoxyribonucleic acid (DNA)
• Sugar/carbohydrate (e.g. glucose, sucrose)
• Fats (fatty acids)

 

Organic substances can be grouped into functional groups and homologous series.  

• Functional group: a group of atoms that gives an organic molecule its unique chemical structure and properties.

• Homologous series: a series or group of organic compounds with the same functional group. 

What are Hydrocarbons?

Hydrocarbons are organic compounds that consist of carbon and hydrogen atoms only. In hydrocarbon molecules, carbon to carbon (C–C) and carbon to hydrogen (C–H) bonds are strong and unreactive. 

There are three functional groups and homologous series for hydrocarbons:

• alkanes
• alkenes
• alkynes

Alkanes

Functional group	Distinguishing feature	General molecular formula	Example
Alkane	C–C	CnH2n+2	ethane

• Alkanes are hydrocarbons consisting of only single carbon-carbon (C–C) and carbon-hydrogen (C–H) bonds

• Alkanes are saturated hydrocarbons because each carbon atom is bonded to a maximum of four atoms

• In alkanes, all carbons are connected to four atoms and therefore exist in a tetrahedral shape with each bond (either C–C or C–H) subtended at a 109.5º angle relative to one another. 

 

Properties of Alkanes

• Alkanes are not chemically reactive

• Alkanes have relatively low boiling, melting points and solubility in water

• Alkanes are non-polar hydrocarbons held together by dispersion forces as its only intermolecular force. Thus, the molecular weight of alkanes largely determines the strength of dispersion force and physical properties such as melting and boiling points.

• Larger alkanes have higher boiling and melting points

• Larger alkanes have lower solubilities in water as molecules become more non-polar

• Straight-chained (linear) alkanes have higher boiling points than branched alkanes of equal molecular weight

 

• Straight-chained structure increases surface area between molecules which in turn increases the strength of dispersion force. Stronger dispersion force gives rise to a higher boiling point

• Alkanes are typically used as fuel e.g. natural gas and fossil fuel to produce large amounts of energy. Smaller, short-chained alkanes combust more efficiently than larger alkanes. For example, the complete combustion of propane (C₃H₈) yield carbon dioxide and water

 

$$2C_3H_8(g) +7O_2(g) \rightarrow 6CO_2(g) + 8H_2O(l)$$

Alkenes

Functional group	Distinguishing feature	General molecular formula	Example
Alkene	C=C bond(s)	CnH2n	ethene

 

• Alkenes are hydrocarbons consisting of at least one carbon-carbon double bond (C=C)

• Alkenes are unsaturated and more chemically reactive than alkanes. This is because C=C bonds are electron-dense and the presence of these electrons allow alkenes to undergo chemical reactions.

• When a carbon atom is connected to another carbon atom via a C=C double bond, it is connected to a total of 3 atoms. This means it exists in a trigonal planar shape with each bond subtended at a 120º angle relative to one another. 

• Alkenes are unsaturated hydrocarbons because the double bond(s) can further react to add more hydrogen atoms. This process of transforming an alkene into an alkane is called hydrogenation.

 

Properties of Alkenes

• Melting and boiling points of hydrocarbons in the alkene homologous series behave in the same way as alkenes. As the number of carbon atoms and the molecular weight of alkenes increase, the strength of dispersion forces increases. As a result, melting point and boiling point increase with size.

• Since alkene molecules have two less electrons than an alkane of the same number of carbons, its dispersion force is slightly weaker. Consequently, the melting and boiling point of alkenes are a few degrees lower than those of alkanes.

Alkynes

Functional group	Distinguishing feature	General molecular formula	Example
Alkyne	C≡C bond(s)	CnH2n–2	ethyne

• Alkynes are hydrocarbons consisting of at least one triple carbon-carbon bond.
• Alkynes, like alkenes, are unsaturated hydrocarbons.
• Carbon atoms in a triple carbon-to-carbon bond are only connected to two atoms. As a result, they exist in a linear shape with each bond subtended at 180º angle relative to one another.

 

 

Properties of Alkynes

• Alkynes are very chemically reactive, more so than alkenes due to the presence of a third covalent carbon-carbon bond in the same location. In other words, triple carbon-carbon bonds are more electron dense and reactive than double carbon-carbon bonds (C=C).

• Physical properties of alkynes are difficult to compare with those of alkanes and alkenes.

• Alkyne molecules can form dispersion forces and, to a slight extent, dipole-dipole forces too.

• Alkynes have higher melting and boiling point than both alkanes and alkenes with the same number of carbon atoms, despite having less electrons. There are a few more factors that contribute to intermolecular force in alkynes:
• The electrons in the triple bond are more easily polarised to create induced dipole-dipole forces which increases the overall attraction between alkyne molecules
• Alkynes have less C–H bonds which means alkyne molecules can move closer together, further increasing the strength of their intermolecular forces.

 

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