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4.1 Basic Concepts and Hydrocarbons - Coggle Diagram
4.1 Basic Concepts and Hydrocarbons
4.1.1 Basic Concepts
Basic Definitions
Hydrocarbon is a compound consisting of hydrogen and carbon only
Saturated: Contain single carbon-carbon bonds only
Unsaturated : Contains a C=C double bond
Molecular formula: The formula which shows the actual number of each type of atom
Empirical formula: shows the simplest whole number ratio of atoms of each element in the compound
General formula: algebraic formula for a homologous series e.g. CnH2n
Structural formula shows the minimal detail that shows the arrangement of atoms in a molecule, eg for butane: CH3CH2CH2CH3 or CH3(CH2)2CH3,
Displayed formula: show all the covalent bonds present in a molecule
Skeletal formula shows the simplified organic formula, shown by removing hydrogen atoms from alkyl chains,
leaving just a carbon skeleton and associated functional Groups.
Aliphatic: a compound containing carbon and hydrogen joined together in straight chains, branched chains or non-aromatic rings
Alicyclic : an aliphatic compound arranged in non-aromatic rings with or without side chains
Aromatic : a compound containing a benzene ring
Naming Organic Compounds
Alkenes
The double bond will be between two carbons. Use the lower
number of the two to show the position of the double bond.
The name for alkenes may include E or Z at start to show the type of stereoisomer
If more than one double bond is present then suffix ends diene or triene. The stem ends in a.
The suffix-en for alkenes can go in front of other suffixes. The alcohol and carboxylic acid groups have higher priority than the alkene group so take precedence with numbering
Haloalkanes
Class the halogen as a substituent on the C chain and use the prefix-fluoro,-chloro,-bromo, or–iodo. (Give the position number if necessary)
Multiple functional group and side chains are listed
in alphabetical order (ignoring any di, tri).
The alkene group has higher priority than the halogenoalkane group so it takes the lowest number on the carbon chain
Alcohols
These have the ending-ol and if necessary the position number for the OH group is added between the name stem and the–ol
If there are two or more-OH groups then di, tri are used. Add the ‘e’ on to the stem name though.
The OH group has a higher priority than the halogenoalkane group and alkene so takes precedence in numbering. The OH is on carbon 1
If the compound has an–OH group in addition to another functional group with a higher priority. The priority group gets the suffix ending and the OH can be named with the prefix hydroxy-:
Aldehydes
An aldehyde’s name ends in–al It always has the C=O bond on the first carbon of the chain so it does not need an extra number. It is by default number one on the chain.
If two aldehyde groups then di is put before–al and an e is added to the stem.
Aldehydes have a higher priority than alcohol so the–OH group uses the hydroxy prefix.
Ketones
Ketones end in-one
When ketones have 5C’s or more in a chain then it needs a number to show the position of the double bond. E.g. pentan-2-one
If two ketone groups then di is put before - one and an e is added to the stem.
Carboxylic Acids
These have the ending-oic acid but no number is necessary for the acid group as it must always be at the end of the chain. The numbering always starts from the carboxylic acid end.
If there are carboxylic acid groups on both ends of the chain then it is called a- dioic acid
Esters
Esters have two parts to their names
The bit ending in–yl comes from the alcohol that has formed it and is next to the single bonded oxygen. The bit ending in–anoate comes from the carboxylic acid. (This is the chain including the C=O bond)
Isomers
Structural Isomers
Definition- same molecular formula different structures (or structural formulae)
Can arise from
•Chain isomerism
•Position isomerism
•Functional group isomerism
Chain Isomers
Compounds with the same molecular formula but different structures of the carbon skeleton
Position Isomers
Compounds with the same molecular formula but different structures due to different positions of the same functional group on the same carbon skeleton
Functional Group Isomers
Compounds with the same molecular formula but with atoms arranged to give different functional groups
Mechanisms
Homolytic Fission
The bond has broken in a process called homolytic fission. Each atom gets one electron from the covalent bond
When a bond breaks by homolytic fission it forms two free radicals. Free radicals do not have a charge and are represented by a dot
A Free radical is a reactive species which possess an unpaired electron
Heterolytic Fission
Heterolytic fission produces ions. One atom gets both electrons
The mechanism
4.1.2 Alkanes
Properties of Alkanes
Formation of alkenes
In alkanesone sp3orbital from each carbon overlap to form a single C-C bond called a sigma σ bond
Boiling Point
Theincreasing boiling points of the alkane homologous series can be explained by the increasing number of electrons in the bigger molecules causing an increase in the size of the induced dipole–dipole interactions (London forces) between molecules.
The shape of the molecule can also have an effect on the size of the induced dipole–dipole interactions (London forces).Long chain alkanes have a larger surface area of contact between molecules for London force to form than compared to spherical shaped branched alkanes and so have stronger induced dipole- dipole interactions and higher boiling points.
Reactivity
The low reactivity of alkanes with many reagents can be explained by the high bond enthalpies of the C-C and C-H bonds and the very low polarity of the σ-bonds present.
Combustion of Alkanes
Alkanes are relatively unreactive due to their non-polar nature and their strong covalent bonds. The electronegativity difference between carbon and hydrogen in alkanes is minimal, leading to an almost equal sharing of electrons and no significant partial charges. This makes it difficult for alkanes to attract nucleophiles or electrophiles.
Alkanes serve as efficient fuels, releasing a significant amount of energy when burnt. They are used in various applications such as power generation, heating, and transportation due to this property.
Alkanes can undergo two types of combustion reactions: complete and incomplete, depending on the oxygen availability.
Complete Combustion
When there is an ample supply of oxygen, alkanes combust completely, forming carbon dioxide and water vapour as products.
Alkanes in their liquid state must be vaporised before combustion.
Smaller alkanes, due to their lower boiling points, vaporise and thus combust more readily.
Larger alkanes have more chemical bonds, hence when combusted, they release more energy per mole, making them better fuels.
Incomplete Combustion
When the oxygen supply is limited, alkanes undergo incomplete combustion, leading to the formation of carbon monoxide and water vapour.
Incomplete combustion may also lead to the production of solid carbon (soot) and the release of unburnt hydrocarbons into the atmosphere.
Reactions of Alkanes with Halogens
Photochemical halogenation of alkanes
Halogens react with alkanes in special light-induced reactions called photochemical reactions.
For the reaction to occur:Ultraviolet (UV) light must be present.
This UV light provides the activation energy to start the reaction
The overall reaction is a substitution, where a hydrogen atom in the alkane molecule is replaced by a halogen atom like chlorine or bromine.
Free Radical Substitution Mechanism
Photochemical halogenation of alkanes follows a three-step free radical substitution mechanism:
Initiation - UV light produces reactive radicals.
Propagation - Radicals react in a chain reaction.
Termination - Radicals join to form stable molecules.
Chlorofluorocarbons and ozone depletion
Chlorofluorocarbons (CFCs) are halogenoalkanes containing chlorine, fluorine and carbon with no hydrogen atoms remaining.
Two examples of CFCs are trichlorofluoromethane (CCl3F) and chlorotrifluoromethane (CClF3).
Ozone (O3) in the upper atmosphere serves as a vital "chemical sunscreen", absorbing harmful ultraviolet radiation from the sun.
CFCs pose a threat to this protective ozone layer, leading to increased risks of sunburn, skin cancer, and damage to plant life.
Problems with Free Radical Substitution
Production of product mixtures
With an excess of halogen, additional substitution reactions can occur.
For example, chloromethane (CH3Cl) can undergo further substitution:
Cl• + CH3Cl ➔ CH2Cl• + HCl
CH2Cl• + Cl2 ➔ CH2Cl2 + Cl•
This results in a mixture of halogenoalkanes (e.g., CH3Cl, CH2Cl2, CHCl3, CCl4) that must be separated.
Formation of multiple isomers
The propagating radical can substitute at any position along a carbon chain.
This leads to the production of various positional isomers.
For example, halogenation of propane yields a mixture of 1-chloropropane and 2-chloropropane.
4.1.3 Alkenes
Properties of Alkenes
They consist of carbon and hydrogen atoms (hydrocarbons).
Each alkene molecule contains at least one carbon-carbon double bond.
The double bond makes them "unsaturated", enabling them to participate in addition reactions.
Bonding in Alkenes
Rotation can occur around a sigma bond
The π bond is formed by sideways overlap of two p orbitals on each carbon atom forming a π-bond above and below the plane of molecule. The π bond is weaker than the σ bond.
Shapes
In alkenes, each carbon atom within the C=C double bond has:
Three regions of electron density.
These regions are arranged in a trigonal planar shape due to equal repulsion between them.
The bond angles around the carbon atoms are approximately 120°.
Reactivity
These nonpolar sigma bonds contribute to low reactivity in alkanes: Their high bond enthalpy makes alkane bonds difficult break.
They do not attract reactive nucleophiles and electrophiles.
Three key features make this pi bond more reactive
High electron density - Makes pi electrons highly accessible for reactions.
Protruding shape - Allows electrophiles to readily attack pi electrons.
Lower bond enthalpy - Requires less energy to break than sigma bonds.
Stereoisomerism
Alkenes are characterised by the presence of carbon-carbon double bonds, which significantly influence their molecular structure:
The atoms connected to each of the doubly bonded carbons are positioned in the same plane as these carbon atoms. This planar arrangement results from the sideways overlap of p orbitals, which forms the π bond.
Around the carbon-carbon double bonds, rotation is restricted due to the stable overlapping of the p orbitals that form the π bond.
It's important to note that while double bonds restrict rotation, single bonds in the molecule can still rotate freely.
Spatial Arrangements
Stereoisomers are compounds that have the same molecular formula and connectivity but differ in their three-dimensional arrangement of atoms.
These isomers typically occur in alkenes where each of the carbon atoms in the double bond has two different groups attached.
E- Z Isomers
Z-isomer - Groups of interest are on the same side of the double bond. "Z" stands for Zusammen, a German word meaning “together”.
E-isomer - Groups of interest are positioned across from each other on opposite sides of the double bond. "E" stands for Entgegen, a German word meaning “opposite”.
Reactions of Alkenes
Electrophilic Addition
Alkenes undergo a type of reaction called electrophilic addition across their carbon-carbon double bond. In this process: The π bond of the C=C double bond breaks.
Atoms or groups add across the carbon atoms.
Addition of Hydrogen
Alkenes react with hydrogen gas (H2) in the presence of a nickel catalyst at 150°C, forming alkanes.
Addition of Halogens
Halogens react with alkenes in an electrophilic addition reaction to form dihalogenoalkanes.
Hydration to form Alcohols
Under the influence of a phosphoric(V) acid catalyst, alkenes react with steam at 300°C and 60-70 atm pressure, undergoing hydration to form alcohols.
Addition of Hydrogen Halides
Hydrogen halides like HCl and HBr add across the C=C double bond in an electrophilic addition, forming halogenoalkanes.
In symmetrical alkenes, the double bonded carbons are identical, leading to a single monosubstituted product.
Polymerisation in Alkenes
Alkenes contain a carbon-carbon double bond. This bond can open up, allowing alkene molecules to join end-to-end and form long chains called polymers
The individual alkene molecules are called monomers.
Joining many monomers together is called polymerisation.
Polymers made this way are called addition polymers because the double bond opens up and monomer units are added to the chain.
Dealing with the accumulation of waste plastics
Incineration
Incinerating waste plastics in specialised facilities can generate useful heat or electricity.
Advantages
Recovers energy from waste plastics, reducing reliance on fossil fuels
Substantially reduces the volume of plastic waste, minimising landfill requirements
Disadvantages
Generates toxic waste products, such as HCl, which require costly removal processes to prevent environmental damage
Incineration is an energy intensive process, partly offsetting the energy recovered
Releases greenhouse gases, primarily CO2, contributing to climate change
Recycling
Recycling waste plastics can be achieved through two main approaches:
Mechanical recycling - This involves sorting, cleaning, and remoulding waste plastics into new objects without significantly altering the chemical structure of the polymer.
Feedstock recycling - This process breaks down polymers into their constituent monomers through cracking. These monomers can then be used as raw materials for the synthesis of new plastics and other organic chemicals.
Advantages
Conserves valuable crude oil resources by reducing the need for raw materials
Typically results in lower CO2 emissions compared to incineration and production from raw materials
Can be more economical than producing new polymers, especially when oil prices are high
Reduces the amount of waste sent to landfills
Disadvantages
Requires complex and costly multi-step processes for sorting, cleaning, and reprocessing waste plastics
Contamination from additives, dyes, or other polymers can compromise the quality and performance of recycled products
Recycled plastics may have limited applications due to reduced mechanical properties and potential contamination issues
Developments in degradable polymers
To address the challenges posed by non-biodegradable polymers, researchers have developed degradable polymers that decompose more quickly, offering environmental benefits by reducing the accumulation of persistent plastic waste.
Biodegradable polymers:
Incorporate starch granules or are derived from plant-based monomers, making them more susceptible to digestion by microorganisms.
Polyamides and polyesters can decompose through acid hydrolysis under the hot, acidic conditions often found in landfills.
Photodegradable polymers
Contain light-sensitive carbonyl groups that absorb UV light, weakening nearby bonds and causing the polymer to fragment into smaller pieces.
Fragmentation accelerates the biodegradation process.
Have limited effectiveness in landfill environments where waste is typically buried, blocking light and preventing photodegradation.