Chapter 10: Organics (part 2 - functional group chemistry) Flashcards
What are the properties of alkanes?
- non-polar molcules
- = insoluble in water
- with only london forces between molecules
- means they are volatile (evaporate easily)
- the first 4 members are gases at room temperature
From a global perspective, what is the most important reaction of alkanes?
combustion
e.g. petrol and natural gas
alkane compounds form the bases of fuels
What does complete combustion of hydrocarbons require and produce?
- requires the presence of excess air/oxygen
- produces carbon dioxide and water
When does incomplete combustion occur and what does it produce?
- occurs when there is a limited supply of air/oxygen
- produces carbon monoxide and soot (C) as well as water
What is the actual equation for the combustion of ethane?
the combination of these three equations (from both complete and incomplete combustion)
What is produced if there is not sufficient oxygen present for complete combustion?
CO2, CO and carbon
How is incomplete combustion dirtier than complete combustion?
incomplete combustion produces a smoky flame and the toxic gas carbon monoxide (which binds to hemoglobin more strongly than oxygen does, so oxygen is prevented from being transported by the blood)
How is the reactivity of alkanes?
apart from combustion, alkanes are generally fairly unreactive
Why are alkanes generally fairly unreactive?
- the high strengths of C-C and C-H bonds means that it is generally energetically unfavourable to break them in a reaction
- C-C and C-H bonds are essentially non-polar and so are unlikely to attract polar molecules or ions
In what conditions do alkanes react with halogens?
in the presence of sunlight or ultraviolet (UV) light
there is no reaction in the dark at room temperature
needed because alkanes are unreactive
What is the equation for the reaction between methane and chlorine in the presence of UV light?
What is the equation for the reaction between ethane and bromine (in temrs of full structural formulas as well)?
What is chloromethane an important contributor to?
to the destruction of the ozone layer
What is a significant source of chloromethane from?
from natural sources such as wood-rotting fungi
What is a substitution reaction?
an atom or a group is replaced by a different atom or group
What reaction is occuring?
a substitution reaction
a hydrogen atom in ethane is replaced by a bromine atom
What is mono-substitution?
the replacement of one hydrogen atom in the molecule by one halogen atom
What is free radical substitution?
a hydrogen atom gets substituted by a halogen (chlorine/bromine)
What are the 3 stages of free radical substitution?
Initiation
Propagation
Termination
What happens in initiation of free radical substitution?
Consider the reaction between methane and chlorine
- breaking apart chlorine molecules into separate chlorine atoms
- Energy needed is provided by the UV light
- The Cl-Cl bond (242kjmol-1) is weaker than the C-H bond (412Kjmole-1) and therefore it is the Cl-Cl bond that is broken
- Cl-Cl bond is broken through a process called homolytic fission (covalent bond breaks, one electorn goes back to each atom that made up the original molecule to form two species that are the same)
- The chlorine atoms produced each have an unpaired electron and is represented by the dot
- they are described as free radicals
- Now there is an increase in the number of free radicals
What is the process of homolytic fission?
when the covalent bond (made up of two electrons) breaks, one electron goes back to each atom that made up the original molecule - in this case to form two species that are the same (‘homo’ means ‘same’)
Which stage does homolytic fission occur in the free radical substitution?
initiation
What are free radicals?
are species (atoms or groups of atoms) with an unpaired electron.
Are very reactive because of this unpaired electron
Why are free radicals very reactive?
because they have an unpaired electron
What is the propagation stage of the free radical substitution?
- free radical collides with a methan molecule in the reaction mixture
- it will combine with a hydrogen atom to pair up its unpaired electron
- now a highly reactive methyl free radical is generated
- this will react with a Cl₂ molecule to form a C-Cl bond
- The Cl* generated in this step can go to react with another methane molecule
- the propagation cycle starts again
How is propagation an example of a chain reaction?
Because one initial event causes a large number of subsequent reactions - with the reactive species being regernated in each cycle of reactions
What happens in the termination stage of the free radical substitution?
- free radicals are always present in very low concentrations so the chance of two colliding is very low
- free radicals do collide sometimes and this brings the chain reaction to an end
- Each termination reaction involves a decrease in the number of free radicals
What is the summary of the free radical substitution in terms of the number of free radicals?
Initiation: increase in the number of free radicals
Propagation: no change in the number of free radicals
Termination: decrease in the number of free radicals
When does the formation of multi-substituted produces occur?
when a chlorine free radical collides with a different molecule in the first propagation step
What is the mechanisms for the free radical substitution reactio nof ethane with bromine?
What are the two possible mono-chlorinated products that can form when butane reacts with chlorine in the presence of UV light?
What is the functional group of alkenes?
C=C
What are the properties of alkenes?
- non-polar molecules = insoluble in water
- only london forces between them = are volatile - the first 4 members are gases at room temperature
Why are alkenes and alkynes unsaturated compounds?
they contain C=C bonds or triple bond C
How is alkenes reactivity compared to alkanes?
alkenes are more reactive
What are 2 reasons why alkenes are more reactive than alkanes?
- the double bond (612Kjmol-1) is not twice as strong as a single bond. The reactions of alkenes usually involve the C=C bond breaking to form a C-C bond (348Kjmol-1). The second component (π bond) of the C=C bond is weaker than a normal C-C single bond (σ bond) and is, therefore, more easily broken
- The double bond (4 electrons) represents a region of high electron density, and therefore it attracts electrophiles
What is an electrophile?
is a reagent (a positively charged ion or the positive end of a dipole) that is attracted to regions of high electron density and accepts a pair of electorns (Lewis acid) to form a covalent bond
What reactions do alkenes undergo?
Addition reactions
What happens in addition reactions?
The molecule X-Y is added across the double bond, which leaves a single bond between the carbon atoms
How is the reaction of an alkene with iodine at room temperature?
very slow
What conditions do alkenes react with halogens?
At room temperature
What conditions do alkenes react with hydrogen?
when heated in the presence of a catalyst (i.e. finely divided nickel) to form alkanes. This reaction is called hydrogenation
What is hydrogenation?
Hydrogenation is an addition reaction where hydrogen molecules are used to saturate organic compounds.
What conditons do alkenes react with hydrogen halides?
by bubbling the alkene through a concentrated solution of the hydrogen halide at 100˚C
What are hydrogen halides?
Hydrogen Halides dissolve in water to give acids. They are diatomic organic compounds with a formula HX, X represents any of the halogens.
Some of the halogens include iodine, chlorine, bromide and fluorine.
How is the reaction of alkenes with HI comparede to HCl?
Reaction with HI is rapid
reaction with HCL is very slow and a catalyst (AlCl₃) is often used for that reaction
What conditions do alkenes react with water?
by passing ethene through concentrated sulfuric acid at room temperature and then warming the product with water
What is called catalytic hydration?
the reaction between and alkene and water
What is catalytic hydration used for?
in the industrial production of ethanol, an important solvent
How is the catalytic hydration process carried out?
by reacting ethene (from crude oil) with steam at 3˚C and 6000kPa pressure with an H₃PO₄ catalust
How can ethanol be produced? (2)
- catalystic hydrattion
- fermentation
How is ethanol produced by fermentation?
- uses a renewable raw material (sugar) and a much lower temperature (although a major energy input can come when ethanol is distilled)
Compare unsaturated fats with saturated fats?
Unsaturated
* C=C bonds present
* liquid at room temp
Saturated fats
* no C=C bonds
* solid at room temp
What process is used in the manufacture of margarine?
hydrogenation of alkenes
What bonds are in a polyunsaturated oil?
many cis C=C bonds (groups either side of the C=C bond are on the same side)
What gives polyunsaturated oil its low boiling point?
the presence of cis double bonds
How is the melting point of margarine increased?
- partial hydrogenation to convert some of the C=C bonds into C-C single bonds
- so its a solid at room temp
What does the partial hydrogenation of margarine produce?
produces trans-fatty acids (groups either side of the C=C bonds are on the opposite sides)
are believed to be bad for health
How can alkenes be distinguished from alkanes?
by shaking them with bromine water
What happens to the bromine water when mixed with an alkene and alkane?
alkane: bromine water no colour change
alkene; bromine water - orange to colourless
How does benzene (C₆H₆) not react like alkenes?
- does not undergo addition reactions under normal conditions
- will not decolorise bromine water
What does benzenereact with chlorine in the presence of a catalyst (i.e. aluminium chloride) to form?
C₆H₅Cl
What is addition polymerisation?
in which a large number of monomer molecules are joined together to form a polymer chain
What undergoes addition polymerisation?
Alkenes
What is polyethene also called?
polythene
What are the conditions for the production of low-density polyethene?
- high temperature
- high pressure
- in the presence of a small amount of oxygen or an organic peroxide
How does the production of high-density polyethene differ from low-density polyethene?
produced at much lower temperatures and lower pressures using a catalyst
How are addition polymers named?
according to the monomer from which they were made
What is the name of this polymer?
Ethene is the monomer
so polyethene is the polymer
How is this an example of addition polymerisation?
because monomer molecules are simply added to each other without anything else being produced
What is PVC - polyvinylchlride more properly known as?
polycholoroethene
How is polychloroethene made by?
the polymerisation of chloroetheme monomers at a moderate temperature and pressure in the presence of an organic peroxide
What is the structure shown in square brackets called?
the repeating unit of polychoroethene
What is the structure shown in square brackets called?
the repeating unit of polychoroethene
When polymerisation occurs, only what is affected?
Only the C=C groups
all the other groups attached to the C=C unit are unaffected
When polymerisation occurs, only what is affected?
Only the C=C groups
all the other groups attached to the C=C unit are unaffected
What is the repeatin gunit for polypropene?
How can the repeating unit for any polymer be identified?
by taking any two adjacent carbon atoms in the main polymer chain
How can the monomer of a polymer be derived?
by simply placing a double bond between the two carbon atoms of the repeating uni
What are the economic importance of polymers?
- most commonly known as ‘pastics’
- have extensive uses in everyday life
- substances are produced in vast quantities by the petrochemicals industry and present a huge environmental problem (using a limited natural source and their disposal after use as they are non-biodegradable)
What is polyethene used for?
making plastic bags, washing-up bowls and bottles
What is polychoroethene used for?
making insulation for electrical cables and window frames
What is polypropene used for?
making car bumpers and carpet fibres
In a plentiful supply of oxygen, what do alcohols burn to produce?
carbon dioxide and water
Which two classes of alcohol can be oxidised?
Primary and secondary
How can a primary alcohol get oxidised into?
Partial oxidation: aldehyde
Complete oxidation: Carboxylic acid
What can oxidising agent can be used to oxidise primary and scondary alcohols?
Acidified potassium dichromate (VI) (K₂Cr₂O₇/H+) or acidified potassium manganate (VII) (KMnO₄/H+)
How to write the partial and complete oxidation equation of a primary alcohol?
use ethanol
Alcohol + oxygen = product + H2O
product (from last equation) + oxygen = a carboxylic acid
What is the oxidising agent?
the Cr₂O₇²-
What is happening in the reducing half-equation in the first reaction of oxidation of a primary alcohol?
Cr₂O₇²- (orange) is reduced to Cr³+ (green)
What is the colour change when acidified dichromate (Cr₂O₇²-) is reduced?
orange to green
What is happening in the oxidising half-equation in the first reaction of oxidation of a primary alcohol?
The alcohol loses 2 H
What is the half-equation for the redcution of manganate (VII) ions to Mn²+?
What is the half-equation for the redcution of manganate (VII) ions to Mn²+?
What is the colour change when manganate (VII) ions is reduced to Mn²+?
Purple to colourless
What is the half equation for the oxidation of ethanal to ethanoic acid?
Its the product of the oxidation of ethan**ol **+ water and turns into a carboxylic acid (COOH)
How doe we get the overall redox equatino for the oxidatino of ethanal by acidified manganate (VII)?
Combine the half equation for the reductino of manganate (VII) ions to Mn²+ and the oxidatino of ethanal to ethanoic acid. then balance the electrons
What is obtained when. areaction mixture containing ethanol and acidifid potassium dichromate (VI) is heated under reflux?
Ethanoic acid is obtained as the main product and the aldehyde is not usually isolated
How can aldehyde from a reflux reaction containing ethanol and acidified potassium dichromate (VI) be isolated?
- Set up the apparatus so that the aldehyde is distilled off as soon as it is formed and before it can be oxidised further
- Aldehydes have a lower boiling point than the equivalent alcohols (and all other compoenents of the reaction mixture) because they do not have a hydrogen bonding between molecules
What are the general changes of the funcitonal groups when primary alcohols are oxidised?
- the OH changes to a double bond O and single bond H = aldehyde (Low BP cuz hydrogen atom is not directly attached to an oxygen atom anymore)
- Add O between the C and H = carbonoic acid (COOH)
How does the oxidation of 2-methylpropan-1-ol change in the oxidation of a primary alcohol reaction?
OH -> COH -> COOH
What are secondary alcohols oxidesed into when heated with acidified potassium dichromate (VI) or accidified potassium manganate (VII)?
They are oxidised to ketones, which cannot be oxidised any further
What is the product of this secondary alcohol udnergoing oxidation?
a ketone
replace the OH and O attached to the C with a double bond O
What does the [O] represent?
oxygen from the oxidising agent
in this case the secon equation the oxidising agent is shown as Cr₂O₇²-
How is this represented in a balanced equation form?
What is the general reaction of a secondary alchol undergoing oxidation? (in terms of functional group)
What product does it give after undergoing the oxidation reaction of a secondary alcohol?
H-C-OH -> C=O (with the other C attached to the C
alcohol to ketone
Do tertiary alcohols undergo oxidation reactions?
No they are resistant to oxidation
What is esterification?
forming an ester by heating an alcohol with a carboxylic acid in thepresence of a small amount of concentrated sulfuric acid as a catalyst
What are the products of esterification?
Ester + water
What is the product of this esterification?
Ethyl ethanoate + water
What is the structural formula of this?
How do the alcohol and the carboxylic acid join together in esterification?
Condensation reaction
The O of the alcohol is joined to the C=O group of the carboxylic acid
One H atom from the alcohol joins with the -OH group from the carboxylic acid - to form water that is eliminated
What can the reaction between an alcohol and a carboxylic acid be described as?
Nucleophilic substitution reaction
Why is esterification also described as a nucleophilic substitution reaction?
Alcohol acts as a nucleophile and substitutes (replaces) the OH group of the carboxylic acid
What is nucleophillic substitution also sometimes known as?
Hydrolysis because it is essentially breaking molecules apart using water
How to draw the structural equation of esterification reactions?
- put the OH groups of the alcohol and the carboxylic acid next to each other
- remove the H from the alcohol and the OH form. the carboxylic acid
- remove them as water and join the C of the carboxylic acid with the O from the alcohol to form an ester
What reactions do halogenoalkanes usually undergo?
substitution reactions
What happens when 1-bromopropane is heated with aqueous sodium hydroxide?
The Br atom is replaced by an OH group to form propan-1-ol
What is the balanced equation and ionic equation of this substitution reaction of halogenoalkanes?
Why is this reaction also known as hydrolysis?
Because it is essentially equivalent to breaking molecules apart using water
Define what is meant by the term nuclephilic substitution?
replacement of atom/group (in a molecule) by a species with a lone pair of electrons/species attracted to an electron-deficient or highly positive carbon atom
What is a nucleophile?
a molecule or negatively charged ion that has a lone pair of electrons - it is attracted to. arelatively highly positively charged region in a molecule. (a region with lower electon density) and donates a lone pair of electrons to form a covalent bond
How do halogenoalkane undergo nucleophilic substitution reactions?
the halogenoalkane is attacked by a nuclephile and the nucleophile replaces the halogen atom
What do halogenoalkanes react with to form amines?
with ammonia
What do halogenoalkanes react with to form nitrils?
with potassium cyanide
What is the functional group of nitrile?
-CN functional group with a triple bond between the carbon atom and the nitrogen atom
How does the -CN change the carbon chain?
The longest chain is now 3 carbons and the name is propanenitrile
How is forming nitrile useful?
provides a way of increasing the length of the carbon chain
Which is more reactive: halogenoalkanes or alkanes?
halogenoalkanes - due. tothe presence of the halogen atom
i.e. chlorine atom (very electronegative) is present in a molecule, the C-Cl bond will be polar and a nucleophile will be attracted to the δ+ carbon atom.
How are halogens reactive?
use chlorine
- very electronegative and if present in a molecule the C-Cl bond will be polar and a nucleophile will be attached to the δ+ carbon atom
- Cl- is classified as a ‘good leaving group’ - the C-Cl bond is weaker than. aC-C bond or. aC-H bond and Cl- is stable in many solvents
