Lec3+4 - Lecture notes 3-4 PDF

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Lec3+4 - Lecture notes 3-4...

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Syllabus ● 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 ● describe the procedures required to safely handle and dispose of

organic substances (ACSCH075) ● examine the environmental, economic and sociocultural implications of obtaining and using hydrocarbons from the Earth

Notes PROPERTIES OF ALKANES:

Intramolecular Bonding In Alkanes: - The intramolecular bonds in alkanes are non polar covalent single bonds.

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The intermolecular bonds between alkanes are very weak dispersion forces. No other force is acting to form an attraction between neighbouring molecules

PHYSICAL STATE AT 25 CELCIUS -

Vaseline is a hydrocarbon with about 30 C atoms in the chain. It is a waxy solid at room temperature. If you leave it in a hot car, the consistency really thins out.

SOLUBILITY, DENSITY AND VOLATILITY -

Volatility is related to dispersion forces as well. The smaller the molecule, the weaker the dispersion force. - Hence, vaporisation occurs at lower temperatures.

SAFETY WITH HYDROCARBONS -

Hydrocarbons, but in particular short chain alkanes, alkenes and alkynes are extremely flammable. At high concentrations they are toxic to humans. The high volatility means that leaking or open containers emit hydrocarbon vapours much more readily than other liquids – this poses an explosion risk.

HOW DO WE SAFELY STORE HYDROCARBONS -

Well maintained cylinders and fittings on gas bottles. C1 – C4 alkanes are commonly used and are always bottled under pressure Addition of odours to propane and butane (LPG) to enable early detection via a sense of smell. Sturdy containers for liquids such as petrol and diesel. Minimal quantities should be stored and used and alkanes kept away from naked flames. Warning signs and clear labelling for all hydrocarbons in storage.

HOW DO WE SAFELY TRANSPORT HYDROCARBONS -

Transportation tankers are made from reinforced steel that can withstand high impacts without rupturing. Petrol tankers on trains and trucks have static electricity dissipation devices to ensure no sparking or earthing that could trigger an explosion. Automobile fuel tanks are located at the opposite end of the vehicle and outside the main shell of the vehicle. Colouration of commonly used fuels aids quick identification in the event of a spill. e.g. – Kerosene is coloured blue. – Unleaded fuel is yellow.

HOW DO WE SAFELY DISPOSE OF HYDROCARBONS -

The correct handling and safe disposal guidelines for dangerous chemicals, including hydrocarbons are shown on each chemical’s MSDS sheet. Organic substances are usually collected and disposed of separately after use in the school laboratory. Small amounts can sometimes be left to vaporise under controlled conditions. Larger amounts are stored in light-dispersive glass-wear and collected by licenced disposal agencies. Empty hydrocarbon containers are environmentally problematic and must also be collected by licenced waste management operators

HYDROCARBONS VS THE ENVIRONMENT -

The impact of hydrocarbons on the environment has been negative: Hydrocarbons have been increasingly used in the last 250 years as a source of energy via their combustion.

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High concentrations of CO2 in the atmosphere are now linked to the combustion of fossil fuels. Global warming is largely due to the mass combustion of hydrocarbons by humans for the last 200 years. Extraction of Crude Oil is environmentally damaging. Heavy industry that is now possible due to the reliable source of energy from hydrocarbon fuels is also a heavy polluter of the environment

HYDROCARBONS VS THE ENVIRONMENT -

The impact of hydrocarbons on the economy has been largely positive: Fossil fuels like coal, oil and gas provide a huge output of energy (heat of combustion) which has provided a reliable source of energy for industry Long-haul transportation industries have boomed – including global travel and aviation The development of the plastics industry which lead to the computing and I.T age Development of advanced drugs and pharmaceuticals for improved quality of life Employment booms relied heavily on the growing manufacturing global industry that relied heavily on the energy for manufacture and the demand for organic compound based products

HYDROCARBONS VS SOCIOCULTURAL FACTORS -

The positive impacts: Development of advanced drugs and pharmaceuticals for improved quality of life. Efficient and cheap form of heating for households. Town electricity supplies possible – extended work hours and increased productivity.

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The negative impacts: Recent studies showing some links to exposure to hydrocarbons and health issues. Increasing cost associated with overfilled landfill and inefficient recycling practices. Normalisation of energy-demanding lifestyles are hard to repeal. Extended work hours and increased productivity.

Syllabus ● investigate, write equations and construct models to represent the reactions of unsaturated(CH 11/12-5) hydrocarbons when added to a range of chemicals, including but not limited to: – hydrogen (H2)

– halogens (X2) – hydrogen halides (HX) – water (H2O) (ACSCH136) ● investigate, write equations and construct models to represent the reactions of saturated hydrocarbons when substituted with halogens

Notes HYDROGENATION -

Alkenes will react readily with hydrogen (H2) gas in the presence of a Nickel catalyst to form an alkane. The double bond ‘opens up’ to accept the H atoms via an addition reaction. Two of these electrons can then ‘break free’ of the bond and become available to bond with other atoms.

Double bond acts as 4 shared electrons

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This process is called hydrogenation and results in the corresponding alkane forming. Ethene therefore undergoes hydrogenation to form ethane.

HALOGENATION -

The Halogens have 7 outer shell electrons and require one more to achieve the octet. Halogens are so reactive that this occurs via addition at room temperature without a catalyst.

HYDROHALOGENATION -

Hydrogen halides are simply molecules that contain 1 hydrogen and 1 halogen atom

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(such as HCl) Hydrogen halides do not require a catalyst to react with alkenes Markovnikov's rule: The hydrogen atom of the HX compound bonds to the carbon with the greatest number of hydrogen atoms in the starting alkane/alkene Product formed using markovnikov's rule is named the ‘major’

HYDRATION -

Alkenes will react with water under specific conditions to form the corresponding alkanol Ethene will form ethanol, butene will form butanol and so on. Phosphoric acid (H3PO4) or sulfuric acid (H2SO4) are commonly used as the acid catalyst The reaction occurs at temperatures around 300° C

Summary table

SATURATED HYDROCARBONS -

Saturated hydrocarbons have to make room for other atoms to substitute onto the molecule and replace existing atoms.

SUBSTITUTION WITH HALOGENS -

Saturated hydrocarbons are much more stable than unsaturated hydrocarbons. This means that unlike alkenes, they will not react spontaneously with halogen molecules. In the presence of U.V light alkanes will slowly react with halogens via substitution with a

H atom.

c

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Only one of the Cl atoms binds to the C atom. The product of the first substitution reaction can then be further reacted to progressively substitute more and more of the original H atoms with halogen atoms.

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If longer chain alkanes undergo substitution with a halogen, the position of the halogen is equally likely on any of the C locants.

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The halogen can hence be rightly placed on any C atom in the chain.

Syllabus ● investigate the structural formulae, properties and functional groups including: – primary – secondary – tertiary alcohols ● explain the properties within and between the homologous series of alcohols with reference to the intermolecular and intramolecular bonding present

Notes PROPERTIES OF ALCOHOLS - BOILING POINT -

The boiling points of alcohols are much higher than their equivalent alkanes.

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This is because the intermolecular forces between neighbouring alcohol molecules are far stronger than their equivalent alkane molecules

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Hydrogen bonding occurs between the OH groups on alcohols. The O atom is far more electronegative than the H atom that it takes on a partial negative charge, The partial negative charge is electrostatically attracted to the H atom on the neighbouring molecule which has in turn becomes partially positive.

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Alkanes have only dispersion forces present as intermolecular forces. The corresponding alkanols have these same dispersion forces as well as hydrogen bonding.

ISOMERS AND THEIR BOILING POINT -

Where the OH group is located on the chain drastically affects the physical properties of the alcohol (such as boiling point).

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The position of the OH group on the carbon chain affects the intermolecular bonding

between molecules and this is reflected in the different boiling points for isomers of alcohols. -

Primary alcohol = Highest BPs Tertiary alcohol = Lowest BPs

EFFECT OF CHAIN LENGTH ON BOILING POINT -

The longer the chain length, the greater the strength of the dispersion forces and therefore the higher the boiling points.

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Chain length small = Lowest BPs Chain length high = Highest BPs

SOLUBILITY OF ALCOHOLS IN WATER -

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Solubility of alcohols decreases as carbon chain length increases. Short chain alcohols like methanol and ethanol are highly soluble in polar water due to the H-bonding that occurs between the highly polar hydroxyl groups on both molecules. Larger alcohols are progressively less soluble in water due to the predominantly non polar nature of the hydrocarbon chain (alkyl tail)....