M7 OC2 anes, enes, ynes

IQ2. How can hydrocarbons be classified based on their structure and reactivity?

2.1 Models of hydrocarbons

2.2 Investigating properties and bonding of organic homologous series

2.3 Shapes of organic molecules

2.4 Properties and bonding of organic homologous series

2.5 Safety and organic substances

2.6 Hydrocarbons from the Earth – Implications

Homologous series are groups of compounds which are characterised by:

 a common general formula

 a common functional group

 similar structures and chemical properties

 gradations in their physical properties (as MM increases)

The shape of the distribution of atoms around any carbon atom usually this will fit one of three types:

Example 1: methane

Example 2: ethane

WHAT HAPPENS IN DOUBLE AND TRIPLE BONDS?

View Videos:

• OC#10 Molecular Shapes https://www.youtube.com/watch?v=6PdKzBrheDk [10.14 mins]

When studying the properties of the hydrocarbons, there are two groups of properties that are important: physical properties and chemical properties.

Chemical properties

In very broad terms, the chemical properties of a compound or a series of compounds are associated with the intramolecular bonding (bonding between atoms in the molecule). Alkanes are characterised by single carbon-carbon and carbon-hydrogen bonds. The paired electrons are arranged in a tetrahedral arrangement and this gives a level of stability to the compounds, especially since the electronegativity values for both carbon and hydrogen are very similar, and so the covalent bonds (and therefore the molecules) are non-polar.

Chemical properties relate to the functional group

Both alkenes and alkynes are chemically more reactive than alkanes due to the presence of a double or triple bond.

Physical properties

The physical properties of the alkanes are related to the intermolecular forces between molecules. As a result the physical properties are not just based on the non-polarity of the covalent bonds within the molecules, but also on their size and shape. The dominant intermolecular bonds are dispersion forces.

The presence of non-polar bonds and non-polar molecules affects the physical properties of the compound. The hydrocarbons are separate (=discrete) molecules with strong covalent bonds in the molecule but only weak dispersion forces between molecules. As the chain length increases, the number of dispersion forces between molecules increases, and hence larger molecules have higher melting and boiling points: small alkanes exist as a gas at SLC (Standard Laboratory Conditions = 25C and 100kPa), butane, octane and several others are liquids, while the longer chains, such as the paraffin waxes (20+ carbon atoms) are solids.

The non-polar nature of the hydrocarbon molecules also makes them insoluble in a polar substance such as water. However as our rule is like dissolves like, they will dissolve in other non-polar solvents, eg CCl4.

The lack of free-moving electrons also means the hydrocarbons are poor electrical conductors.

Intermolecular Forces Revisited

First, a review of intermolecular forces (attractions) as opposed to intra-molecular bonding.

Van der Waals forces is a general term used to define the attraction of intermolecular forces between molecules. There are two kinds of Van der Waals forces

• weak London Dispersion Forces
• stronger dipole-dipole forces.

1. Dispersion Forces

The chance that an electron of an atom is in a certain area in the electron cloud at a specific time is called the "electron charge density." Since they do not all stay in the same area 100 percent of the time, if the electrons all go to the same area at once, a dipole is formed momentarily. Even if a molecule is non-polar, this displacement of electrons causes a non-polar molecule to become polar for a moment.

Since the molecule is momentarily polar, where the electrons are concentrated at one end, the molecule is partially negatively charged on that end. This negative end makes the surrounding molecules have an instantaneous dipole also, repelling the neighbouring molecules' electrons and attracting the surrounding molecules' positive ends. This process is known as the London Dispersion force of attraction.

When the molecules become temporarily polar, the melting and boiling points are raised because it takes more heat and energy to break these bonds. Therefore, the greater the mass, the more electrons present, and the more electrons present, the higher the melting and boiling points of these substances.

2. Dipole-Dipole Forces

These forces occur in molecules that are permanently polar versus momentarily polar. In this type of inter-molecular interaction, a polar molecule such as water or H2O attracts the positive end of another polar molecule with its negative end of its dipole. The attraction between these two molecules is the stronger dipole-dipole force.

3. Hydrogen "bonding"

A hydrogen bond is a special type of dipole-dipole attraction which occurs when a hydrogen atom bonded to a strongly electronegative atom is close to another electronegative atom with a non-bonding pair of electrons.

These bonds are generally stronger than ordinary dipole-dipole and dispersion forces, but weaker than true covalent andionic bonds.

Introduction

For a hydrogen bond to occur there must be a hydrogen atom covalently bonded covalently bonded with a strongly electronegative atom such as N, O, or F. The electronegative ion or molecule must posses a non-bonding electron pair (O has two in water, shown as twin sets of dots in the diagram below left) in order to form a hydrogen bond.

Since the N, O or F atom is strongly electronegative, it pulls the covalently bonded electron pair closer to its nucleus, and away from the hydrogen atom. The hydrogen atom is then left with a partial positive charge, creating a strong dipole-dipole attraction (shown as dotted lines in the diagrams below) between the hydrogen atom bonded to the donor, and the non-bonding electron pair on the electronegative atom.

Note: Chlorine has a relatively low ability to form hydrogen bonds because it is a large atom. When the radii of two atoms differ greatly in size or are both large, their nuclei cannot get close enough when they interact, so they have only a weak interaction.

SUMMARY

Looking at the graph above, there is a clear trend of increasing boiling point.

The trend line is almost perfectly linear. It suggests a strong relationship between boiling point and molar mass for alkanes. The difference between each member of the alkanes is the addition of a CH2 group. This additional group increases the size of the molecule and also increases the strength of the dispersion forces between the molecule. These are the forces which must be overcome for the molecules in a liquid to be free as gas molecules.

View Videos:

• OC#9 https://www.youtube.com/watch?v=vyBn-MOzkVU [11.23]
• OC#11 https://www.youtube.com/watch?v=OSHWS9-vLwk [14.09]

View Videos:

• OC#12 https://www.youtube.com/watch?v=tB4cMfqbnOU&list=PLeFSFSJ9WqSDRKZVcK33Gtz60RsJ5_t7Q&index=18 [7.07]

Hydrocarbons are commonly found in high concentrations in fossil fuels. Two of the simplest hydrocarbons are methane and ethane, the primary components of natural gas.

Petroleum is a critical commodity in the modern world. What sorts of compounds can be obtained from a barrel of crude oil? How are they obtained?

ENVIRONMENTAL:

• Obtaining:

Obtaining petroleum and natural gas from the Earth involves drilling through rocks deep in the Earth’s crust. Hydrocarbons from the drill machine’s lubricants can be dispersed into surrounding water polluting the surrounding seawater or ocean. These hydrocarbons are toxic to aquatic organisms that reside in the sea.

These rocks, which often contain barium ions from the traces of lubricant that remain, are also returned into the ocean. These barium ions interfere with enzyme activities, which can result in death of living organisms.

Potassium ions in the machine lubricants in extracting the hydrocarbons can result in uncontrollable algae growth, leading to eutrophication. This allows algae to grow on the water surface, blocking the sunlight reaching to the plants beneath the water as well as oxygen gas that is dissolved in the water. This can result in the death of plants, which decreases the oxygen availability in the water for aquatic animals like fish, as well as bacteria that further use more oxygen to decompose dead plants. It destroys the aquatic ecosystem, turning a habitable environment into a toxic one for the original species.

Another environmental implication would involve the noise pollution through the sending of sound waves to detect potential hydrocarbon deposits for drilling. This would disturb local aquatic organisms, such as whales, as well as any humans that are near the area. The sound waves can disorientate whales, resulting in large scale whale stranding on beaches and can cause death.

Hydrocarbons must be transported by ship from the sea to land to oil refinery so that petroleum can be split into components. Oil spillages have also caused environmental damage similar to those already mentioned, but hydrocarbons can be washed ashore polluting beaches.

Using:

Hydrocarbons that enter the human body could cause severe respiratory irritation. The long term effects of hydrocarbon exposure is also currently not fully understood.

Along with the expansion of the petrochemical industry from energy generation to polymer production and the associated economic benefits of both industries, environmental considerations have become more critical, especially as scientists link climate change to our reliance on the burning of fossil fuels. This has spawned new industries devoted to greener energy production and investigations into alternate fuel sources which might reduce the impact on the environment.

• Burning of hydrocarbons release carbon-dioxide in the atmosphere which is in fact, a greenhouse gas. Thus extensive use of hydrocarbons increases the risks of greenhouse effect.
• Incomplete burning of hydrocarbons results in the formation of carbon monoxide, a fatal gas that can cause death within minutes to exposure.
• CFC, a widely used hydrocarbon in refrigerators and spray cans when exposed to the environment causes harm to the ozone layer and thus, is a contributing factor to the greenhouse effect. It also causes exposure of humans and animals to harmful UV rays.
• Aldehydes and other toxic materials released from burning plywood inhibit photosynthesis in plants, cause eye and lung irritations, and even possibly cause cancer.
• Benzene molecules have been found to deplete red blood cells, cause cancer in mammals and damage bone marrow.
• Through potential implications such as leaking toxic hydrocarbons, potassium ions and eutrophication as we have discussed earlier as an environmental implication, they could all result in the death and reduction in biodiversity of aquatic organisms. It is important to preserve biodiversity for many countries as aquatic organisms serves a major source of economic revenue.

Economic:

Potential implications such as leaking toxic hydrocarbons, potassium ions and eutrophication discussed earlier could all result in the death and reduction in biodiversity of aquatic organisms. It is important to preserve biodiversity as aquatic organisms serve as a major source of economic revenue in many countries.

The world's reliance on a small number of sources of petroleum puts their financial viability at risk should there be shortages from natural causes, embargoes or war.

Sociocultural:

Workers involved in the drilling process to obtain hydrocarbons will be exposed to drilled rocks that are covered in toxic hydrocarbon lubricants, as well as the lubricants in the machinery itself, which can be accidentally or voluntarily inhaled as oil mists in the air.

The leaking of toxic hydrocarbons, potassium ions and eutrophication during the extraction of hydrocarbons can result in a decline in biodiversity of aquatic organisms. This has implications in removing or limiting the range of available food which humans can enjoy. As the supply of aquatic organisms decreases due to hydrocarbon pollution, the price of seafood would increase, it would be less affordable for the global population in general.

The presence of toxic hydrocarbons would result in our everyday potable (drinkable) water derived from the sea being toxic. The treatment of water would need to be more extensive, which would incur additional cost to consumers.

View Videos:

• OC#13 https://www.youtube.com/watch?v=n_vU5H-dPhw&list=PLeFSFSJ9WqSDRKZVcK33Gtz60RsJ5_t7Q&index=19 [8.02]