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Chemistry 1.7

AS edexcel chemistry - introduction to organic

What is the difference between a molecular formula and an empirical formula? A molecular formula shows the exact number of atoms of each element present in a molecule, while empirical formula shows the simplest whole number ratio of atoms of each element in a molecule.
What is the difference between a displayed formula and a skeletal formula? Displayed formula- diagram showing relative placing of atoms and number of bonds between them; skeletal formula -diagram uses lines to represent carbon backbone (C atoms and H atoms bonded to C atoms aren't drawn, H atoms bonded to other atoms are drawn)
What is a structural formula? A list of conventional groups, where brackets indicate a branch off the preceding carbon atom
What is a homologous series? A group of compounds that have the following features: shared general formula, shared functional group, similar chemical properties, physical properties show a gradation, molecular formulae of consecutive members differ by CH2
What is a functional group? A group of atoms bonded to the carbon backbone that give the members of a homologous series similar chemical properties
List all functional groups + prefix/suffixes of: 1) alkanes, 2) alkenes, 3)alcohols, 4) halogenoalkanes, 5) ketones, 6) aldehydes, 7) carboxylic acids 1) all single bonds, -ane 2) C-C double bond, -ene 3) OH, hydroxy- -ol 4) chloro, bromo, iodo + -ane 5) C=O in middle of chain, oxo-, -one 6) C=O at end of chain, -al 7) COOH at end of chain, -oic acid
What happens in nomenclature when there are two or more of the same substituent? State the numbers of carbon atoms that they are bonded to, then use di/tri/tetra e.g. 2,4,4- trimethylpentane
What happens in nomenclature when there are multiple functional groups? One goes as the prefix and the other as the suffix - preference for prefix is alcohol < ketone < aldehye < carboxylic acids. Alkene suffix can fit before other suffixes e.g. butenal - alkene + aldehyde.
What is hazard? The intrinsic danger associated with working with particular substances or apparatus
What is risk? The likelihood that hazard will cause harm, reduced with safety precautions
How can risk be reduced? (always state hazard then mitigation in exam) risk assessment, working on a smaller scale, less hazardous chemicals e.g. dilute, fume cupboard for toxic/volatile chemicals, keep lids on toxic/ volatile chemicals when not in use
What is the general formula and functional group of the alkanes? CnH2n+2. Saturated hydrocarbons that only contain single bonds
What are the physical properties of the alkanes? * Boiling point increases as no. of C atoms increase due to stronger intermolecular forces. Branching reduces points of contact and attraction between molecules so lowers boiling point. Difference decreases as mass increases by a smaller % each time.
Continued * Melting point increases as no. of C atoms increase. * Insoluble in water as they are non polar but dissolve in organic solvents and in each other
Are alkanes relatively reactive or unreactive and why? Unreactive due to strong sigma C-C and C-H bonds, and they are non-polar
Explain the combustion of the alkanes Good fuels because they burn well in oxygen and reaction is very exothermic. Complete combustion - sufficient oxygen to oxidise carbon, products are CO2 and H2O. Incomplete combustion - insufficient oxygen, products are CO2, CO, C (soot), H2O
What is free radical substitution and what is a free radical? An endothermic chain reaction involving free radicals (atoms or groups of atoms with an unpaired electron so very reactive)
Is chlorination or bromination of the alkanes faster? Chlorination because Cl radical is more reactive than Br radical
Explain general mechanism of free radical substitution of alkanes Initiation: UV light provides energy for homolytic fission of covalent bond between halogens, two free radicals. Propagation: free radical hits another molecule, producing a new free radical. Termination: two free radicals hit each other and are removed
Draw the mechanism for the bromination of ethane Products are bromine, bromoethane, and butane
What are isomers? Molecules with same molecular formulae but different structures
What type of isomerism is present in the alkanes and what are the isomers of the first 5 alkanes? Structural isomerism - branching of the carbon chain. Methane, ethane and propane have 1, butane has 2, pentane has 3.
How are alkanes obtained? * fractional distillation of crude oil- vaporised, similar boiling points piped off * cracking of higher boiling fractions - catalytic (lower temp +pressure, catalyst, carbocations, cycloalkanes), thermal (high temp +pressure, free radical substitution)
What is a problem with use of alkanes as fuels? * crude oil is a finite source, will eventually run out despite increasing demand. * there is a theory that the increase in atmospheric CO2 (due to combustion of fossil fuels) is causing climate change
How can the release of CO2 emissions be lowered? * use alternative energy sources e.g. renewables, biofuels * recycle and reuse products as it requires less energy than extracting or making more * low emission or electric cars
What is the general formula and functional group of alkenes? CnH2n. Unsaturated hydrocarbons. Functional group: one C-C double bond consisting of a sigma bond (head-on overlap of two atomic orbitals) and a pi bond (sideways overlap of two p orbitals)
What types of isomerism is there of the alkenes? Structural - branching of carbon chain. Positional - location of functional group changes. Geometric - there is restricted rotation of double bond and at least two different substituents on the carbon atoms involved in the double bond
What types of geometric isomerism is there? Cis/trans: 2 different types of substituents on C atoms, cis same groups on same sides, trans same groups on opposite sides across double bond. E/Z- 3 or 4 different types of substituents, * atom on left and right with higher mass, Z same side, E opposite
Are alkenes more or less reactive than alkanes and why? More reactive due to weak pi bond which allows addition reactions to take place (where two substances react together to form a single substance). Pi bond breaks, leaving a sigma bond between 2 carbon atoms, and each can form a sigma bond with a new atom
Explain the combustion of the alkenes Burn with a smoky flame when ignited in oxygen becuase each moleucle contains a smaller proportion of hydrogen than corresponding alkane.
Explain the hydrogenation of the alkenes Alkenes react with hydrogen in an addition reaction to form the corresponding alkane in the presence of a nickel catalyst at 150ºc - used in the hardening of polyunsaturated vegetable oils into margarine
What is halogenation of the alkenes (reaction, reactants, products)? Alkenes react with halogens via electrophilic addition - pi bond breaks, 2 sigma bonds form. Halogen (chlorine or bromine in liquid form) is electrophile- species that accepts 2 electrons to form a new covalent bond. Produces di-substituted halogenoalkane
How do alkenes react with hydrogen halides? They react with gaseous hydrogen halides through electrophilic addition - H is electrophile as H halides are polar (H has slight + charge). Fastest with HI as H-I bond is weakest. Markovnikoff's rule used when alkene is asymetrical
What is markovnikoff's rule? When a gaseous hydrogen halide reacts with an asymmetrical alkene e.g. propene, the hydrogen bonds to the carbon atom that already has the most hydrogens attached. Forms the most stable carbocation (ion with + charge on carbon atom)
Explain the general mechanism for electrophilic addition with hydrogen halide 1) Pair of electrons in pi bond move to H atom, dative covalent bond, forming carbocation. 2) heterolytic fission of H-halogen bond, both electrons go to halogen, producing an anion. 3) lone pair of electrons on anion move to carbocation, dative covalent
Draw the mechanism for the electrophilic addition of hydrogen bromide to propene Major product is 2-bromopropane, minor is 1-bromopropane.
What are the two tests for alkenes? * reaction with bromine water (electrophilic addition, brown to colourless) * reaction with potassium manganate (VII) solution, oxidation ot produce alkane diol
Explain the reaction of alkenes with bromine water Electrophilic addition, so brown colour of bromine water goes colourless. Carbocation is more likely to react with water (solvent, a nucleophile) than bromide ion, giving an alcohol. E.g. bromine water + ethene -> 2-bromoethanol
Explain the reaction of alkenes with potassium mangnate (VII) solution Oxidation takes place so alkane diol forms (2 OH groups). If solution is acidified with dilute sulphuric acid, purple colour of manganate (VII) ions go colourless. If neutral, purple manganet (VII) ions form brown precipitate of manganese (IV) oxide.
How do polymers form? Addition reaction- one alkene molecule breaks the π bond in another alkene molecules so they become attached
What are the conditions for polymerisation of alkenes? High temperature to break pi bond and high pressure so alkene molecules come into close contact
Draw the monomer unit for but-1-ene, 1-chloroprop-1-ene
What is a polymer? A long chain containing thousands of repeating units called monomers
Created by: 11043



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