IB Chemistry - Organic Chemistry
Structural isomers
Are compounds with the same molecular formula but different structural formula. They therefore have different chemical and/or physical properties
Definition of optical isomers
Are non-superimposeable mirror images of each other. The molecule must posses a chiral centre. It is an enantiomer.
Definition of enantiomers
Are non-superimposeable mirror images of each other. They are molecules that are not identical to their mirror image. Optical isomers are enantiomers. Enantiomers are said to be chiral and optically active.
Alkynes
Are unsaturated hydrocarbons that contain a triple carbon to carbon bond.
What are the arenes?
Arenes are a class of compounds that are derived from benzene C₆H₆. They form a class of compounds known as aromatics, which have properties distinct from other organic compounds.
What is the functional group for an arene?
Arenes contain the phenyl funtional group C₆H₅.
Trends with increasing hydrocarbon chain, and explanation
As the length of a hydrocarbon chain increases, so doing boiling point. As the molecular mass/chain length/number of electrons increases, the amount of dispersion forces also increases
Design a reaction pathway using balanced chemical equations, for the conversion of propene to propanoic acid
As we need to form a carboxylic acid, we need to form a primary alcohol. Issue is with converting via H₂O will create 2-propanol, which we don't want. Therefore we need to go via a halogen. 1. Propene + H₂ → propane - Electrophilic addition - Ni - Heat 2. propane + Cl₂ → 1-chloropropape + HCl - Free radical substitution - UV-light 3. 1-chloropropane + NaOH → 1-propanol + NaCl - Sn2 4. 1-propanol + [o] → propanoic acid - Oxidation - Cr₂O₇²⁻ - H⁺ - Reflux
What reactions do benzene undergo?
Benzene can undergo electrophilic substitution reactions in which one or more hydrogen is replaced by an incoming group. Benzene undergoes electrophilic substitution reactions. An electrophile is a reactant that are themselves electron deficient. That is they have a positive charge and see the benzene ring as an electron donor.
What are the conditions of the nitration of benzene? Give and example of the equation of the reaction.
Benzene is warmed with a mixture of concentration nitric acid and concentration sulfuric acid. In this mixture there is an equilibrium resulting in the formation of the nitronium ion (NO₂⁺). HNO₃+2H₂SO₄↔NO₂⁺+2HSO₄⁻+H₃O⁺ C₆H₆+NO₂⁺→C₆H₅NO₂+H⁺
Alkenes
Consist of a carbon backbone that contains at least one carbon to carbon double bons. They are called unsaturated hydrocarbons.
Alkanes
Consist of carbon and hydrogen only connected by single bonds. They are called saturated hydrocarbons.
What does the rate of formation rely on in the above reaction?
Forming the carbocation is the slowest step. Sn1 reactions are best conducted using protic, polar solvents. Sn1 reaction depends only on the concentration of the halogenoalkane. Eg. rate = k[halogenoalkane]
Naming carboxylic acids
Functional group always at the end of the chain, therefore no need for numbers Ending will be -oic-
Unsaturated hydrocarbons
Hydrocarbons with multiple carbon - carbon bonds
Saturated hydrocarbons
Hydrocarbons with only single carbon - carbon bonds
What are common electrophiles?
H⁺ (from a hydrogen halogen) and X₂ (with x referring to a halogen)
What will be the major product formed between ICl and propene?
ICl is polarized as I⁺ Cl⁻ owing to the greater electronegativity of Cl than I. So when it undergoes heterolytic fission it will form I⁺ and Cl⁻. By application of Markovnikov's rule, the I⁺ will attach to the terminal carbon, while Cl⁻ will add to carbon 2. The product is therefore 1-iodo-2-chloropropane.
How can benzene become a phenylamine?
If nitrobenzene undergoes reduction.
What is the mechanism called for an alkene to go to a halogenoalkane, and what are the conditions?
- Electrophilic addition - HX - Heat
What is the mechanism called for an alkene to go to an alcohol, and what are the conditions?
- Electrophilic addition - H₂O - 300°C - H₃PO₄
What is the mechanism called for an alkene to go to a dihalogenoalkane, and what are the conditions?
- Electrophilic addition - X₂
What is the mechanism called for an alcohol and carboxylic acid to go to an ester, and what are the conditions?
- Esterfication - c. H₂SO₄(l) - Heat
What is the mechanism called for an alkane to go to a halogenoalkane, and what are the conditions?
- Free radical substitution - X₂ - UV-light
What is the mechanism called for an halogenoalkane to go to an alcohol, and what are the conditions?
- Nucleophilic substitution - NaOH
What reactions do alcohols undergo?
1. Combustion 2. Oxidation 3. Esterfication
How do you assign priorities to substituents?
1. Consider each of the double bond carbons separately, identify the two atoms directly attached and rank them according to atomic number. 2. E is when the priority groups are on the opposite side (like trans) 3. Z is when the priority groups are on the same side (like cis)
Physical properties of alkanes
1. Gas at room temp 2. Does not dissolve in water 3. Colourless 4. Does not react with acids/bases 5. Does not react with oxidising agents
Design a pathway for the formation of the compound 2-propyl ethanoate. Use alkenes as the only organic starting material.
1. Identify that 2-propyl ethanoate is an ester. Made from an alcohol and a carboxylic acid. Therefore we need 2-propanol and ethanoic acid. 2. Propene → 2-propanol - Electrophilic addition - H₂O - 300°C - H₃PO₄ 3. Ethene → ethanol → ethanal → ethanoic acid - First is an electrophilic addition (same as before) - Oxidation (Cr₂O₇²⁻/H⁺/warm) - Oxidation (Cr₂O₇²⁻/H⁺/reflux) 4. 2-propanol + ethanoic acid → 2-propyl ethanoate + water - Esterfication - c. H₂SO₄ (l) - Warm
When naming organic compounds remember to
1. Identify the longest backbone of carbon atoms 2. Side chains are numbered with the lowest number
How many chiral carbons in glucose?
4 different chiral centres. It is very optically active.
What is always added in a nucleophilic substitution?
A Sodium Hydroxide group -NaOH
Definition of a chiral carbon
A carbon joined to four different atoms or groups that are tetrahedrally arranged around it. Called an asymmetric centre. Carbon atom is indicated with an asterisk.
What does optically active signify?
A compound can rotate the place of polarised light as it passes through a solution of the compound.
How are ketones reduced? How do you write the equation?
A ketone can be reduced to a secondary alcohol cue the addition of LiAlH₄ or NaBH₄. You use the latter for these equations as it is cheaper and better for the environment. Eg CH₃CH₂COCH₃→(above LiAlH₄ or NaBH₄) CH₃CH₂CHOHCH₃
Definition of a racemic mixture
A mixture of two enantiomers in equal amounts and is optically inactive
Outline how a polarimeter can be used to detect whether or not a compound is chircal
A polarimeter is used to determine whether a beam of plane-polarized light has been rotated or not. A chiral compound rotates the plane of plane-polarized light in either a clockwise or anti-clockwise direction
Naming amines
A primary amine is when the NH₂ group is on the end of a chain, the naming goes the stem goes first followed by amine A secondary amine is when the NH₂ group is a branch on the chain, the naming goes the number, then amino, then the stem A tertiary amine is when there is a methyl group in the compound, the naming goes the number, then amino, then methyl, then the stem
In the oxidation of a primary alcohol to a carboxylic acid what if I want to obtain the intermediate product, the aldehyde?
A process called distilation can be used to extract this product from a mixture of two or more chemicals. The aldehyde has a distinct temperature, hence there is the physical property of boiling point
What is the difference between protic and aprotic solvents?
A protic solvent is involved in Sn1 mechanisms. It is a polar solvent and can form hydrogen bonds. It is also able to donate a proton. An aprotic solvent is involved in Sn2 mechanisms. It is a non-polar solvent but cannot form hydrogen bonds. It also cannot donate a proton.
Definition of a nucleophilic substitution reaction
A reaction between a halogenoalkane and sodium hydroxide
Homologous seriers
A series that has the same general formula. A series of compounds which consists of similar chemical properties and differ from each other by CH₂. Their physical properties show trends
What are monomers?
A small unit made to form a polymer. They contain a double carbon-carbon bond
What is a substitution reaction?
In substitution reactions one or more of the hydrogen atoms in an alkane is replaced by a different atom or functional group. This involves the breaking of the C-H bonds and making new bonds with the substituting atom or group. This type of reaction is called free radical substitution.
What is the process of initiation?
In the presence of UV-light sufficient energy is avalible to break the Cl-Cl bond to produce two chlorine radicals eg. Cl₂→(Uv-light)→2Cl∙
What does the rate of formation rely on in the above reaction?
It depends on the identity of the halogen. Sn2 reactions are best conducted using aprotic, non-polar solvents. For Sn2 it is dependant on the concentration of both the hydroxide ions and the halogenoalkane. Eg. rate = k[OH]⁻[halogenoalkane]
Definition of an Sn2 reaction
It is a nucleophilic bimolecular substitution reaction. Sn2 is a concerted reaction with a transition state.
Definition of an Sn1 reaction
It is a nucleophilic unimolecular substitution reaction. Involves a carbocation intermediate.
Naming ketones
Ketones require at least three carbons. The functional group will NOT be at the end of the chain A number is required if there is more than one possibility (from pentane onwards) The ending is -one-
Density
Mass per unit of volume. Calculated as mass divided by volume.
List first six alkanes
Methane, Ethane, Propane, Butane, Pentane, Hexane
Which is the stronger nucleophile and why, NH3 or H2O?
NH3 as it is the less electronegative, thus its hydrogen atoms are less "tightly held" by the nitrogen.
What is the functional group for amines?
NH₂
Which is the stronger nucleophile and why, OH- or H2O?
OH-, has a negative charge
Which is a better nucleophile and why, hydroxide or water?
OH⁻ is a better nucleophile (i.e. a lower activation energy) because it have a negative charge and three lone pairs of electrons.
What catalyst and conditions are required for an alkene to undergo an addition reaction?
Phosphuric acid (H₃PO₄) issues as a catalyst along with 300°C
Physical and chemical evidence for the structure of benzene
Physical - look at bond lengths and enthalpie's Chemical - Only likes to undergo substitute reactions. Does not like to undergo addition reactions.
Discuss the similarities and differences between physical and chemical properties on enantiomers
Physical properties will be the same with one exception. The molecule will interact differently with plane polarised light. It may rotate the plane polarised light to the right (positive) or to the left (negative). Chemical properties will be the same with one exception. The way the molecule reacts with other optically active isomers. For example biological substrates.
What are polymers?
Polymers are very large covalent molecular substances containing tens of thousands of atoms. They are formed by joining together a great many small molecules called monomers in a process known as polymerisation.
Distinguish between the three types of alcohols
Primary alcohols is where the OH group is located between the end of the chain. Secondary alcohols is where the OH group is in the middle of the chain, and the carbon the OH is attached to is also attached to on hydrogen. Tertiary alcohols is where the OH group is located in the middle of the chain, but the carbon the OH is attached to is not connected to a hydrogen.
What does the reaction combustion involve?
Products are always CO₂ and H₂O
Describe the structures of propane and cyclopropane and describe their difference in their molecular formula.
Propane: C₃H₈ Cyclopropane: C₃H₆
General formula for ketones
R-CO-R'
General formula for amines
RNH₂ where r is the alkyl
What is the most common type of substitution reaction?
Reactions of alkanes that involve halogenation (Cl, F, I, Br)
What does an ester do in real life?
Responsible for some of the natural and synthetic flavours and smells in ice-creams, lilies, flowers and fruits. Also solvents.
What happens with cis-trans isomerism in cycloalkanes?
Result of a restricted rotation around carbon-carbon bonds. This lack of rotation also occurs at the C=C bond in alkenes, giving rise to the possibility of cis-trans isomerism in substituted alkenes.
Identify which of the of three isomers of C4H9Br will react with aqueous sodium hydroxide almost exclusively by an SN1 mechanism. Explain the symbols in the term SN1.
SN1 is characteristic of tertiary halogenoalkanes. SN1 stands for substitution, nucleophilic, unimolecular.
What are ketones reduced to?
Secondary alcohol
Which halogenoalkanes undergo which nucleophilic substitution reactions?
Sn1 = tertiary Sn2 = primary Secondary can undergo both.
Functional groups
Something that is attached to the hydrocarbon chain that plays an important role in the chemistry of organic molecules. They are the reactive parts of a molecule.They determine the chemical properties of the homologous series, and the chain length, which causes the variation in physical properties Fluoro, Chloro, Bromo etc. are apart of the halogenoalkanes
Substitution reaction and products
Takes place under UV light. A hydrogen atom is replaced by a halogen. If one halogen is with Carbon then it is just the stem. If two halogens are with Carbon then it is di before the stem. If three halogens are with Carbon then it is tri before the stem. If four halogens are with Carbon then it is tetra before the stem.
How are tertiary alcohols oxidised?
Tertiary alcohols are not oxidised by potassium dichromate
Explain Markovnikov's rule and how it is applied
The dominant product/major product of an electrophilic addition reaction of a hydrogen halide is when the hydrogen atom attaches itself to the carbon that had more hydrogen atoms at the start. The formation of the major product has a cation that is more stable than the minor product. Eg. propene, hydrogen will attach to the CH₂ furthest to the left and the halogen will attach to the CH in the middle.
What occurs with an addition reaction?
The double bond is broken and water adds across the double bond
What is the process of termination?
The process is terminated when two radicals react eg. Cl∙+Cl∙→(UV-light)→CL₂ CH₃∙+CL∙→(UV-light)→CH₃Cl CH₃∙+CH₃∙→(UV-light)→C₂H₆
What is the process of propagation?
The radicals then react with methane molecules. In the process a methyl radical is produced. The methyl radical reacts quickly with a Cl₂ molecule. eg. Cl∙(g) +CH₄(g)→(UV-light)→CH₃∙(g)+HCl(g) CH₃∙(g)+Cl₂(g)→(UV-light)→CH₃Cl(g)+Cl∙(g) This is a chain reaction
Using the formula RBr to represent a bromoalkane, write an equation for the rate-determining step of the reaction above.
The rate-determining step in the SN1 mechanism involves only the single reactant, the halogenoalkane. It breaks its carbon- halogen bond heterolytically forming two ions. RBr → R+ + Br-
Why does electrophilic addition occur in two stages?
The reaction occurs in two stages and is initiated by species known as electrophiles. These species will attack a molecule at a region of high electron density, such as a double or a triple bond by acting a Lewis acids and accepting an electron pair.
Naming ethers
The shortest chain (when cut by oxygen) is said first with the ending of th or t (pent or meth), then oxy, then the longest chain is written as an alkane
Describe the process of the reduction reaction from nitrobenzene to a phenylamine.
Two-stage reaction. Conditions are a tin catalyst and hydrochloric acid. The initial reaction produces the amine salt. Draw nitrobenzene. Arrow with above Sn/ c. HCl. Goes to form Benzene with circle, with NH₃⁺ coming off. This is called the phenyl ammonium ion. The amine has to be generated from this by the addition of excess alkali. Draw the phenyl ammonium ion. Plus OH⁻. Goes to form Benzene with NH₂ coming off. Plus H₂O. This is called phenylamine.
What catalyst is required for a substitution reaction with halogens?
UV light
Definition of stereoisomers
Where the molecule has atoms attached in the same order but differ from each other in their spatial or three dimensional arrangement. Subdivided into two classes, conformational isomers and configurational isomers
When does cis-trans and E/Z isomerism occur?
Where there is restricted rotation around atoms
What are the three stages of a substitution reaction known as a reaction mechanism
1. Initiation (photochemical homolytic fission) 2. Propagation 3. Termination
For each of the following determine whether stereoisomerism is possible: 1. But-1-ene 2. 2,3-dibromobut-2-ene 3. 1-chloroprop-1-ene 4. 2-chloroprop-2-ene
1. No 2. Cis and trans are possible 3. Cis and trans are possible 4. No
Additional rules when naming organic compounds are:
1. Numbers are separated by a coma 2. Numbers and names are separated by a dash 3. Words/names have no spaces between them 4. If there is more than one side branch or group, then they are listen alphabetically
Why are members of the alkane series unreactive?
1. The relative bond enthalpies between C-C and C-H bonds. Strong bonds =large amount of energy required to break. 2. The polarity of the C-C and C-H bonds. C-C non-polar covalent tends to be left alone by other molecules. C-H polar covalent, however C can leave H for another halogen.
Alcohols
Alcohols contain the OH functional group. The organic molecule changes to ol.
What are carboxylic acids reduced to?
Aldehyde, then primary alcohol.
Chemical properties of alkanes
Alkanes are generally insoluble in water, but they will dissolve in non-polar solvents
Explain carefully why alkenes undergo electrophilic addition reactions. Name the product of the reaction between but-2-ene and bromine.
Alkenes have a double bond which is an electron-dense region and so is susceptible to attack by electrophiles which are themselves electron deficient. They undergo addition reactions because they are unsaturated. The product is 2,3-dibromobutane.
State of alcohols
Always liquid never aqueous
Definition of electrophile
An electron deficient species that can accept electron pairs from a nucleophile. Electrophiles are Lewis acids.
Explain what is meant by the term nucleophile
An electron-rich atom or groups of atoms that is attracted to a slightly positive carbon atom. The strength of the nucleophile depends on its charge and electronegativity.
Which halogenoalkane has the lowest rate of formation?
Bromine, (fluorine is the worst), as it has a lower bond enthalpy.
Which compound reacts most readily by a SN1 mechanism? A (CH3)3CCl B CH3CH2CH2CH2Cl C (CH3)3CI D CH3CH2CH2CH2I
C
What is the functional group for aldehydes
CHO
Naming aldehydes
CHO is always at the end, so no numbers. Simply write stem and add -anal-
Which is the stronger nucleophile and why, CH3O or OH-?
CH₃O, less electronegative
Which is the stronger nucleophile and why, OH- or CN-?
CN⁻, less electronegative
What is the functional group for ketones?
COC
What is the functional group for an ether?
COC (where oxygen is in the middle)
What is the functional group for an ester?
COO (both an aldehyde and ketone)
What is the functional group for carboxylic acids?
COOH
Describe the reaction mechanism for an Sn1 reaction
Called Sn1 as there is one reactant in the initial step, only one molecule is involved in formed the reaction intermediate (the carbocation). To form the reactive intermediate the halogenoalkane undergoes heteroytic fission. 1. Draw a curly arrow from positive C to negative halogen. 2. Carbocation involves the origional tertiaty halogeoalkane with only 3 bonds to the carbon, which adopts a positive sign. Plus halogen which has a negative sign. 3. Redraw carbocation on new line. 4. OH⁻ ion comes in attacking carbon directly from the side that the halogen used to be on. Draw a curly arrow from negative O to positive C. 5. Draw tertiary alcohol.
Other functional groups needed (two)
Carbon double bonded to oxygen with one bond to nitrogen (which is bonded to hydrogen) - peptide Carbon triple bonded to nitrogen - nitrile
How are carboxylic acids reduced? How do you write the equation?
Carboxylic acids can be reduced to a primary alcohol (vie the aldehyde) by the addition of LiAlH₄. This is the only chemical that can reduce an acid. Very expensive! Eg CH₃COOH→(above LiAlH₄)→CH₃CHO→(above LiAlH₄) CH₃CH₂OH
When does optical isomerism occur?
Chirality exists where there is an asymmetric carbon atom
Description of a substitution reaction
Chloroethane is produced by heating mixtures of chlorine gas and ethane. Alternatively the reaction can be initiated by exposing the mixture to UV-light. The reaction conditions cause the Cl₂ molecules to break into ∙Cl radicals which attack the Carbon-Hydrogen bonds in ethane. This is described as free-radical substitution.
Example of nucleophilic substitution
Chloromethane is converted to methanol when reacted with OH ions
How do you determine between 'cis' and 'trans' isomers?
Cis isomers are when the thing that is being attached, eg. a halogen or methyl group, are both on the top or both on the bottom. Trans isomers are when the thing that is being attached, one is on the top and the other is on the bottom.
General formula for aldehydes
Cn(H2n+1)CHO
General formula for Alkenes
CnH2n
General formula for alcohols
CnH2n+1OH
General Formula for Alkanes
CnH2n+2
General formula for alkynes
CnH2n-2
Kinds of combustion reactions and products formed
Complete combustion (excess oxygen), products are carbon dioxide and water Incomplete combustion (limited oxygen), products are carbon monoxide,carbon and/or water vapour
Hydrocarbons
Compounds containing carbon and hydrogen
How do you draw a benzene ring?
Draw a hexagon and every second line draw another line inside the hexagon.
How is a benzene ring formed?
Each carbon is bonded to two other carbons and one hydrogen, leaving one delocalised electron in a p orbital. The orbitals overlap creating delocalised electrons in a ring. This causes the benzene ring to be very stable.
What is the mechanism for alkenes undergoing addition reactions?
Electrophilic addition
What is a type of reaction can benzene undergo?
Electrophilic substitution, more specifically nitration
How are esters formed?
Esters are made by a condensation reaction between a carboxylic acid and an alkanol, where water is lost.
Properties of an ester
Esters composed of small compounds are volatile and smelly and have low boiling points. Esters of large molecular size have a higher boiling point and have less volatility
Substitution reaction stems - functional groups
F = fluoro Cl = Chloro B = Bromo
Describe how to draw a reaction between ethene and hydrobromic acid.
First step is the attack of the H⁺ electrophile on the ethene molecule. The H⁺ will attack the double bond, so curly arrow from bond to H⁺. This will go to form a carbo cation of CH₃CH₂. The carbon on the right will have a positive sign. Second step is that now a carbon atom has a positive charge it can be attacked by a nucleophile, which provides an electron pair to form the new bond. The bromide ion in the solution is a nucleophile and it attacks the carbocation to form bromoethane. Draw bromide ion with a lone pair of electrons and negative charge. Curly arrow from bromide arrow, around a hydrogen to go to the carbon with positive charge. This will go to form CH₃CH₂Br.
Naming esters
First, with the alkanol, take away the anol and replace with yl. Second, with the carboxylic acid, replace to ate with ic. eg. Methanol Propanoic acid → Methyl Propanoate
Describe the difference in geometry between a compound that exhibits geometric isomerism and one that exhibits optical isomerism
Geometric: - C=C or cyclic molecule - Molecules attached differ from each other in space - Must break bonds to superimpose Optical: - Chiral carbon - Mirror image, non superimposable
Describe the mechanism for an Sn2 reaction
In this reaction the hydroxide ion and the halogenoalkane need to form a reactive intermediate. In this reaction initially the nucleophilic OH⁻ is attracted to the halogenoalkane and attacks the slightly positive carbon atom that is bonded to the halogen. Note that Sn2 requires an inversion of configuration at the carbon, (LOOK AT DIAGRAM), therefore the OH⁻ ion group will attack the carbon from the opposite side to where the halogen is attached. The halogen takes up a large space around the carbon atom, so the OH has to attack from the opposite side. 1. Draw a curly arrow from the O molecule with a negative sign to the positive C 2. Draw a curly arrow from C to the halogen which has a negative sign. A reactive intermediate is formed in which the OH and the halogen are both partially bonded to the carbon. 3. Draw out primary halogenoalkane. The two hydrogens are attached horizontally to the C. The halogen is attached below the C with dotted lines. The OH is attached above the C with dotted lines. The halogen and OH must be separated by two hydrogens. 4. Draw brackets around this with a negative sigh because of the OH ion. This quickly results in the halogen being displaced by the OH ion producing a primary alcohol. 5. Draw primary alcohol and halogen separately with the halogen having a negative sign.
Addition reactions
Involves the breaking of double carbon-carbon bonds to form a saturated hydrocarbon.
Definition of cis-trans isomers
Isomers that occur in alkanes or cycloalkanes, and differ in the positions of atoms relative to a reference plane.
Condensed structural formula
It contains the minimum information needed to describe to molecule. Eg. Propane C3H8, would be CH3CH2CH3.
HL HL HL HL HL HL HL HL HL HL HL HL HL HL HL HL HL What is nucleophilic substitution and when does it occur?
Once a more negative atom such as chlorine has been substituted for a hydrogen atom in an alkane, the molecule becomes polar. The carbon is said to be electron deficient. This makes the carbon atom at the other end of the bond susceptible to attack by negatively charged ions known as nucelophiles. A nucleophile is an electron rich species containing a lone pair of electrons which is donated to electron deficient carbon.
How are secondary alcohols oxidised?
One step process 1. Alcohol to ketone with the use of [O], heat and reflux
Isomers of benzene
Only one isomer exists for compounds such as 1,2 dibromobenzene. As it is symmetrical all adjacent positions in the ring are equal.
How do you draw the nitration of benzene? Explain the process of the electrophilic substitution.
The NO₂⁺ acts as an electrophile, attacking the benzene ring to produce an intermediate carbocation in which the delocalisation in the benzene pie electrons are disrupted. Draw benzene ring (with circle) with one hydrogen coming off. Plus symbol and NO₂⁺. Draw a curly arrow from the circle (coming from near the H coming off) to the NO₂⁺. This goes to the carbocation where there are two lines coming from the benzene with H and NO₂. There is a plus symbol in the middle with a dotted U. The carbocation can eliminate the hydrogen, restoring the delocalised pie electrons, resulting in a nitrobenzene molecule. Draw benzene with circle. The NO₂ coming off the benzene. Plus H⁺. This is called a mono substituted benzene ring because only one H had been substituted.
Why does benzene react with electrophiles?
The benzene ring, like the double bond, has a high electron density owing to the presence of pie-electrons in the ring. This means that it can be attacked by electrophiles and form a carbon cation intermediate. Pie bonding is located at the double bond. The delocalised electrons are located in the centre of the benzene ring.
What is the main process of polymerisation?
The breaking of the double carbon-carbon bond to just a single bond
Describe how to draw a reaction between ethene and bromide.
The bromide ion will approach the double bond. The bromide closest to the double bond will have a partially positive charge, while the bromide further away will have a partially negative charge. A curly arrow is drawn from the double bond to the positively charged bromide. Another curly arrow is drawn from the positively charged bromide to the negatively charged bromide. This forms the carbocation CH₂BrCH₂, with the carbon on the right adopting a positive charge. Plus Br⁻ with a lone pair of electrons. The second step follow the other mechanism.
Why are alkenes more reactive than alkanes?
The double bonds are more likely to break than C-H bonds and are therefore more reactive
Suggest explanations for the following: (a) Iodo- and bromo- compounds are more useful than chloro- compounds as intermediates in synthesis pathways.
The iodo- and bromo- compounds are more useful because they are more reactive than the chloro- compounds due to their weaker carbon-halogen bonds. This means that they can react with a wider range of compounds and give rise to a range of intermediates and products.
Suggest explanations for the following: (b) Two compounds X and Y have the same molecular formula, C4H9Cl. When each compound is reacted with dilute alkali and AgNO3(aq) is added, a white precipitate that darkens on exposure to air forms rapidly with X, but only slowly with Y.
The substitution reaction of OH for Cl occurs in both these compounds, displacing Cl⁻ which forms the white precipitate of AgCl, which darkens on exposure to air. The tertiary halogenoalkane C(CH₃)₃Cl isomer reacts more quickly than the primary isomer CH₃CH₂CH₂CH₂Cl because it undergoes an Sn1 mechanism, which is faster. X must be the tertiary halogenoalkane because it reacts more quickly, due to its Sn1 mechanism. Y is the primary halogenoalkane, which reacts more slowly due to its Sn2 mechanism.
Describe how the identity of the halogen in a halogenoalkane influences the rate of the Sn2 reaction
The weaker the bond between the halogen and the carbon to which it is bonded in the halogenoalkane, the greater will be the rate of reaction. Hence Iodine gives the greatest rate of reaction as it is the least electronegative.
How are primary alcohols oxidised?
There is a two step reaction. 1. Alcohol to aldehyde with the use of [O] or Cr₂O₇⁻²(dichromate) and heat 2. Aldehyde to carboxylic acid with the use of [O], heat and reflux
Why does cycloalkanes and non-cyclic alkenes undergo geometric isomerism?
There is no rotation possible around the carbon atom.
Definition of conformational isomers
They interconvert by rotation about a 'o' bond. Single C-C bonds, therefore bonds can be simply rotated around to form another isomer.
Definition of configurational isomers
They interconvert only by breaking and reforming a bond. They have a permanent difference in their geometry. Configurational isomers are further subdivided into cis-trans and E/Z isomers and optical isomers. They have the same chemical formula but a different arrangement of atoms in space.
Viscosity
Thickness of a liquid
What is the above reaction's mechanism called? Why?
This reaction is classified as electrophilic addition, because the bromine forms an induced dipole, which ultimately breaks apart to Br⁺ and Br⁻, with the first being the electrophile.
What is the above reaction's mechanism called? Why?
This reaction is classified as electrophilic addition, because the first step involves attack by an electrophile (H⁺).
What reaction does alkenes undergo?
Unsaturated alkenes can undergo addition reactions with water to produce alcohols.
How can a neutral halogen act as an electrophile?
Upon approaching the double bond the movement of electrons in the halogen-halogen bond causes an induced dipole. The halogen will ultimately undergo hetrolytic fission forming a X⁺ and X⁻ ion.
In the oxidation of a primary alcohol to a carboxylic acid what if I want to collect the carboxylic acid only?
Using a technique called reflux we can ensure that all the aldehyde will be converted to the carboxylic acid.
Volatile molecules
Volatile molecules turn into gases easily. That is they have weak intermolecular forces that can be broken with small amounts of energy. A highly volatile molecule is one that has very low intermolecular forces of attraction (dispersion). A molecule with a low volatility requires a large amount of energy to break intermolecular forces(hydrogen bonding).
When is E/Z isomerism used?
When more than two substituents are bonded.
Explain how enantiomers are optically active
When plane polarised light is passed through a sample of the enantiomers it will rotate the place polarised light. Rotation of the light to the right is given a positive sign. Rotation to the left is given a negative sign.
How do you know when an alcohol undergoes an oxidation reaction?
When there is a change in colour from red, to orange to green. This is the changing colour of dichromate
Definition of heterolytic fission
When two non-bonding electrons stay with the more electronegative of the two bonding atoms.