Benzene is a naturally occurring aromatic structure, which is very stable planar ring structure with delocalised electrons.
Define a model.
a model is a simplified version that allows us to make predictions and understand observations more easily.
What homologous series is benzene in?
Benzene is the simplest member of the arene homologous series.
What are benzene/'s empirical and molecular formula?
It has the molecular formula C6H6 and therefore an empiricle formula of CH.
What are the uses of benzene?
It is a liquid at room temperature and is a key ingredient added to gasoline as it increases the efficiency of the car engine.
Describe Kekules model of benzene.
Although the empirical and molecular formula of benzene has been known since the 1850's, the structure was difficult to determine.
Friedrich August Kekule was German chemist who, in 1865, suggested that benzene was a six-membered ring with alternating single and double bonds between the carbon atoms.
What evidence led kekule to creating his model?
He discovered that when one group was added to benzene, only one isomer was ever made; but when two groups were added; there was always three structural isomers produced.
What were the problems with Kekulé's model?
There are three pieces of experimental evidence that do not support the Kekulé model:
- Unlike alkanes, benzene is resistant to addition reactions.
- Enthalpy of hydrogenation of benzene shows that benzene is much more stable than was predicted.
- All six carbon bonds in benzene are the same length.
How did Kekule explain how benzene did not undergo similar reactions to alkenes?
Ethene will readily undergo addition reactions but benzene tends to undergo substitution of a hydrogen atom rather than addition reactions. Kekulé tried to explain this by saying the double and single bonds changed position in very fast equilibrium.
Describe how the enthalpy of hydrogenation of benzene shows that benzene is much more stable than predicted, and therefore is a problem with Kekules model?
Hydrogenation is the addition of hydrogen to an unsaturated chemical. Using bond enthalpy data we can calculate the enthalpy change for the complete hydrogenation of cyclohexane and cycle-1,3,5-hecatriene. Cycle-1,3,5-hexatrene is he Kekulé model of benzene. However, experimentally it is found that the enthalpy change for hydrogenation of benzene is -208kJ mol-1, which shows it is 152 kJ mol-1 more energetically stable than predicted.
Describe how the bond lengths of the carbon - carbon bonds in benzene is a problem with Kekules model?
X-ray diffraction techniques have shown that all six carbon bonds in benzene are 0.140 nm, which is between a C-C single bond at 0.147 nm and a C=C double bond at 0.135 nm. Kekule's structure suggests that there should be three shorter C=C double bonds and three longer C-C single bonds. This evidence disproved the Kekule structure as all six bonds are the same length.
Describe the delocalised structure of benzene.
It is now thought that benzene has a delocalised electron structure. The delocalised model can explain all three pieces of experimental evidence that do not support Kekules rule.
In the delocalised structure, each of the six carbon atoms donates one electron from its p-orbital. These electrons combine to form a ring of delocalised electrons above and below the plane of the molecule. The electrons in the rings are said to be delocalised as they are able to move freely within the ring and do not belong to a single atom. Therefore unlike Kekules structure, all bonds in this ring are identical, so they are the same length.
What does the delocalisation of electrons in benzene lead to benzene being?
The delocalisation of electrons leads to benzene about 152 kJ mol-1 more stable than expected when using the kekulé model. Becuase so much energy is needed to disrupt this delocalisation, benzene is very stable and resistant to addition reactions.
Define a substitution reaction.
a substitution reaction is where a group or atom is exchanged for another group or atom in a chemical reaction.
Define benzene derivative?
A benzene derivative is a benzene ring that has undergone a substitution reaction.
Define a benzene derivative.
A benzene derivative is a benzene ring that has undergone a substitution reaction.
What type of reaction will benzene undergo?
As benzene has delocalised electrons it ios energetically more stable than initially calculated. So it rarely undergoes addition reactions but it will undergo substitution reactions. In these reactions, a hydrogen atom is substituted with a different group and a benzene derivative is formed.
What are the common groups that will substitute a hydrogen atom on a benzene ring?
Single-substituted benzene derivative. What is the name of this compound?
- Stem - the longest chain of carbon atoms is the aromatic ring and has the stem benzene.
- Prefix - there is one ethyl group on the ring so there is no need to number.
How would you name a double-substituted benzene derivative?
(two hydrogen atoms on different cari=bons of benzene can be replaced by two groups)
- If the groups are different, the name is written in alphabetical order. One group will be added first and be given carbon number 1.
- When naming double-substituted benzene derivatives, the smallest possible numbers should be used.
Define electrophilic substitution.
Electrophilic substitution is a substitution reaction where an electrophile is attracted an electron-rich atom or part of a molecule and a new covalent bond is formed by the electrophile accepting an electron pair.
Define a reaction mechanism.
A reaction mechanism is a model with steps to explain and predict a chemical reaction.
What does the electron dense structure of benzene mean it is susceptible to?
Name the reaction steps that can be associated with electrophilic substitution.
- Electrons above and below the plane of atoms in the benzene ring attract an electrophile.
- The electrophile accepts a pair of π electrons from the delocalised ring and makes a covalent bond. This is the slowest step and known as the rate-determining step.
- A reactive intermediate is formed where the delocalised electrons have been disrupted.
- The unstable intermediate releases an H+ ion and the stable product is formed. This is a very fast step.
Summarise a reaction mechanism using curly arrows, for the electrophilic substitution in benzene.
What is nitration?
This is an electrophilic substitution reaction where a hydrogen atom is exchanged for a nitro group (-NO2).
What is the reagent and catalyst for the nitration of benzene?
For the nitration of benzene, the reagent is concentrated nitric acid, with concentrated sulphuric acid as a catalyst.
Summarise the nitration of benzene with a symbol equation.
C6H6 + HNO3 → C6H5NO2 + H2O
Describe the process of carrying out a nitration reaction.
Initially, the concentrated nitric acid and concentrated sulphuric acid are mixed together in a flask held in an ice bath. Then benzene is added and a reflux condenser is set up, keeping the mixture at 50 oC to prevent further substitution reactions occurring.
Draw the mechanism for the nitration of benzene.
In the reaction, the sulphuric acid is needed to generate the NO2+ electrophile from the nitric acid. The sulfuric acid is regenerated after the nitration and is, therefore, a catalyst.
HNO3 + H2SO4 → NO2 + HSO4- + H2O
Describe the halogenation of benzene.
Benzene does not directly react with halogens as the aromatics ring is too stable. A halogen carrier such as iron (which forms an iron halide in situ), iron halides or aluminium, halides are used. The halogen carrier will generate a positive halogen ion.
What can halogen carriers be used for the halogenation of benzene?
How does FeBr3 react to form an ion that can act as an electrophile in the halogenation of bromine?
Bromine can react with iron (III) bromide to form a positive bromide ion that can act as an electrophile. This can be represented by the following balanced chemical equation:
Br2 + FeBr3 → Br+ + FeBr4-
The Br+ is generated in situ. It can then attack the benzene ring and electrophilic substitution occurs.
Draw the halogenation of benzene with Br+ (from FeBr3)
What happens to the halogen carrier after halogenation?
The halogen carrier is a catalyst and regenerated at the end of the halogenation, as released H+ from the benzene ring forms HBr. The following balanced chemical equation illustrates this:
FeBr4- H+ → HBr + FeBr3
Define a Friedel-crafts reaction.
A substitution reaction where hydrogen is exchanged for an alkyl, or acyl, chain.
Why does bromine water decolourise in the presence of an alkene?
Shaking ethene with bromine water will cause decolourisation as the coloured bromine is iused to form the colourless 1,2-dibromoethane.
Draw the pie bond present in ethene.
Describe the relative resistance of benzene to bromination compared with alkenes.
The delocalised electron density of the π-system in the benzene compared with localised electron density of the π-bond in alkenes
When non-polar molecules like bromine approach the benzene ring, there is not enough electron density between the carbon atoms to induce a dipole and start the reaction. This is also the case when attempting to substitute alkyl halides like haloalkanes. (by using a halogen carrier a stronger electrophile can be generated and alkylation can occur)
Explain the bromination of cyclohexane.
If cyclohexene is mixed with bromine ater, an addition reaction occurs. The first part of the mechanism is the bromide molecule having an induced dipole-dipole due to the interaction of the π-bond of the cyclohexane.
Who developed Friedel-Crafts and when?
In Paris during the 19th century, French Chemist Charles Friedel worked with James Crafts, an American chemist, to develop a technique for aromatic electrophilic substitutions where hydrogen is substituted for an alkyl chain.
How does a Friedel-Crafts reaction occur?
This occurs by breaking a C-H bond and forming a C-C bond and is called alkylation. It is very difficult to add alkyl groups to benzene and this was a significant breakthrough in organic synthesis. In all these reactions a strong Lewis acid is used as a catalyst. (accepts a pair of electrons)
How does Friedel-Crafts happen for haloalkanes?
Haloalkanes like chloromethane are mixed with a halogen carrier such as iron (iii) chloride. The halogen carrier acts as a catalyst and is regenerated at the end of the reaction. A reactive carbocation is made which undergoes electrophilic substitution with the benzene ring.
Show the general mechanism for a chloroalkane undergoing a Friedel-Crafts reaction, where R is any alkyl group.
This reaction will occur at room temperature.
What is a problem with the atom economy of Friedel-crafts of haloalkanes?
Multiple substitutions are likely and therefore a mixture of products is made. The products can be separated using fractional distillation or chromatography. The actual yield of a singly-substituted product can be improved by adding excess benzene.
What is the mixture of products produced in the Friedel-Crafts reaction with haloalkanes caused by?
The mixture of products is caused as each successiv substitution maked the dlocalised π-electrons more nucleophillic and therefore more susceptable to electrophillic attack. This increase in reactivity is due to the alkyl chain donating electrons top the aromatic ring.
Apart from haloalkanes what else can the Friedel-Crafts reaction happen with?
What is the functional group of an acyl chloride?
An acyl chloride has the functional group of RCOCl and is very reactive.
Describe the Friedel-crafts reaction with acyl chloride.
Acyl chloride can be used in a Friedel-crafts reaction as the halogen carrier to substitute just one hydrogen atom. As the carbonyl group withdraws electrons from the aromatic ring, a less reactive ketone is made. So, only one substitution can occur.
What are the conditions for the Freidel-Crafts reaction of acyl chlorides?
The reactions are called acylation. The reaction mixture is held at about 60oC for 30 minutes under reflux, for the reaction to occur.
Show the acylation using displayed formula, including the conditions.
Define a phenol.
Phenols are a class of aromatic compounds where a hydroxyl group is directly attached to the aromatic ring.
When does a hydroxyl group attached to an aromatic ring not become a phenol derivative?
If the hydroxyl group is attached to an alkyl chain on the aromatic ring then the compound is no longer a phenol derivative. It would be described as an aromatic alcohol.
Why is phenol a weak acid?
Phenol is a weak acid, as it partially dissociates in water. The chemical equilibrium can be shown in a balanced chemical equation:
C6H5OH + H2O ⇔ H3O+ + C6H5O-
As a weak acid, phenol will react with strong bases. Write a balanced symbol equation for the reaction between phenol and sodium hydroxide.
As a weak acid, phenol will react with strong bases to form salt and water.
C6H5OH + NaOH → C6H5O-Na+ + H2O
What makes phenol a weak acid?
Phenol is an acid because it reacts with strong bases such as NaOH. however, it is a weak acid because it does not react with carbonates. Phenol will not react with weak bases such as sodium carbonate.
Why is phenol more reactive than benzene?
Phenol is more reactive than benzene. This is due to the p-orbital electrons from the oxygen of the hydoxyl group adding to the delocalised electrons of the aromatic ring. So the π-system of the aromatic ring becomes more nucleophillic. The increase in electron density allows the aromatic ring in phenol to be more susceptible to electophillic attack as it can induce a dipole in non-polar molecules.
Why can phenol undergo direct halogenation, unlike benzene?
As the aromatic ring in phenol is more electron-dense, it can induce a dipole in the non-polar bromine molecule. So phenol can undergo direct halogenation, unlike benzene.
Define the directing effect.
The directing effect is how a functional group attached directly to an aromatic ring affects which carbon atoms are more likely to undergo substitution.
Describe the enhanced reactivity of phenol in comparison to benzene.
Phenol will readily undergo electrophilic substitutions with a variety of reagents without the presence of a catalyst. This enhanced reactivity is due to extra electrons from the oxygen on the hydroxyl group being donated to the π-system of the aromatic ring.
What is the bromination of phenol?
Phenol will undergo a triple substitution reaction with bromine water at room temperature.
The resulting product is a white precipitate of 2,4,6-tribromophenol, which smells of antisceptic.
What is the balanced chemical equation for the bromination of phenol?
C6H5OH + 3Br2 → C6H2Br3OH + 3HBr
Explain the nitration of phenol.
- Phenol will undergo a single-substitution reaction with dilute nitric acid (HNO3) at room temperature. This reaction forms a mixture of 2-nitrophenol and 4-nitrophenol.
- Unlike the nitration of benzene, this reaction does not require a concentrated nitric or sulphuric acid catalyst.
- However, if concentrated nitric acid is used, a triple substitution reaction occurs forming 2,4,6-trinitrophenol.
What is the balanced symbol equation for the nitration of phenol with dilute HNO3?
C6H5OH + HNO3 → C6H4(NO2)OH + H2O
Draw the molecule formed in the nitration of phenol with concentrated HNO3?
The position of substitution, substitution reactions can be more favourable for certain carbon atoms when the functional group is directly attached to the aromatic ring.
Describe the position of substitution in phenol.
In phenol, the hydroxyl group pushes additional electrons into the π-system. This makes substitution reactions mainly occur on the 2 and 4 positions of the aromatic ring. The hydroxyl group activates these carbon atoms so their rate of substitution is faster than the other positions. This is known as the 2- and 4- directing effect. This effect is more pronounced in aromatic compounds with an NH2 group directly attached to the aromatic ring.
Describe the position of substitution seen when -NO2 groups are directly attached to the aromatic ring.
When -NO2 groups are directly attached to the aromatic ring a 3-directing effect is seen. The nitro-group withdraws electrons from the π-system and makes the rate of substitution highest on the third carbon atom.
What are the uses of understanding the positions of substitutions in aromatic compounds?
In organic synthesis, a reaction pathway can be designed to maximise the desired product. By considering the electron donating or withdrawing effects of a directly attached functional group, predictions can be made as to the positions of which substitutions will take place.
An electron donating group (e.g. OH) on the aromatic ring causes a ......... directing affect and a ......... species as a product.
Please fill in the blanks.
An electron donating group (e.g. OH) on the aromatic ring causes a 2- and 4- directing effect and a 2,4,6 triple substituted species as a product.
An electron-withdrawing group (e.g. NO2) on the aromatic ring causes a ......... directing affect and a ...... product.
Please fill in the blanks.
An electron-withdrawing group (e.g. NO2) on the aromatic ring causes a 3- directing affect and a single substituted product.
Define a nucleophile.
A nucleophile is a species attracted to an electron deficient part of a molecule, where it donates a pair of electrons makes a new covalent bond.
What is a carbonyl functional group?
A carbonyl functional group is C=O.
What molecules can be formed from a carbonyl functional group?
If the functional group is on the end carbon atom, an aldehyde is formed; if the carbonyl is not at the end of a carbon chain, then a ketone is formed. Each class of compounds has different properties.
Describe the oxidation of aldehydes.
Aldehydes will undergo oxidation to form a carboxylic acid. the reagents are potassium dichromate, K2Cr2O7, and sulphuric acid, H2SO4. Reagents react in situ to form the oxidising species Cr2O72-, and H+.
In the laboratory, the reaction mixture is gently heated under reflux. As the reaction proceeds, a colour change is observed, from orange to green, due to the oxidation state of the chromium changing.
What is the balanced ionic equation for the oxidation of ethanal?
The balanced ionic equation for the oxidation of ethanal to ethanol is;
3CH3CHO(l) + Cr2O7 2-(aq) + 8H+(aq) → 3CH3COOH(aq) + 2Cr3+(aq) + 4H2O(l)
What is the simplified balanced symbol equation for the oxidation of ethanal?
CH3CHO + [O] → CH3COOH
What makes carbonyl compounds susceptible to nucleophilic addition reactions?
carbonyl compounds have a dipole in the C=O functional group. This makes them susceptable to nucleophillic attack on the 𝛿+ carbon atom.
How does a nucleophilic addition reaction occur in carbonyl compounds?
A nucleophile donates a lone pair of electrons to the electron-deficient carbon. Simultaneously, the π-bond in the C=O breaks and a reactive intermediate is formed. The extra electron pair is quickly donated to the neighbouring hydrogen to form an alcohol group and the stable product.
Nucleophilic addition reactions for carbonyl groups can be modelled in a general mechanism, where R is the alkyl group or hydrogen, H-A is a small molecule like H2O or HCN, and Nu- is a nucleophile.
Draw this general mechanism on a carbonyl compound.
What is sodium tetrahydroborate?
- Sodium tetrahydroborate(III) is more commonly known as sodium borohydride and has the formula NaBH4. It is a reducing agent commonly in organic synthesis.
- This compound is made of a BH4- ion, which acts as a source of hydride ions, H-. The hydride ion is the species that is involved in the electrophilic addition and reduction of carbonyl compounds to alcohols.
How can the reducing agent sodium tetrahydroborate(III) be summarised?
Draw the mechanism for the nucleophilic addition of propanal with ethanol and a reducing agent.
What is hydrogen cyanide?
Hydrogen cyanide is a weak acid that will partially ionise in solution. A cyanide nucleophile with a negative charge on the carbon atom is formed.
Give the balanced symbol equation for the ionisation of hydrogen cyanide.
HCN + H2O ⇔ CN- + H3O+
(other sources of cyanide include sodium cyanide, NaCN)
How does hydrogen cyanide react with carbonyls?
The cyanide ion cannot react directly with carbonyl compounds. But, when the reaction is acidified, the carbonyl functional group becomes more reactive as the polarity of the C=O bond is increased.
Draw the reaction mechanism of the nucleophilic addition reaction of hydrogen cyanide with propanone.
The reaction can be summarised as a mechanism. In the second step, the hydrogen could also be obtained from another small molecule in the reaction mixture such as ethanol.
The addition of cyanide allows further carbon atoms to be added to the organic molecule. In this reaction, a hydroxynitrile is formed, which is an important chemical used in many industrial processes.
Why does hydrogen cyanide need to be reacted in low acidity solutions?
- Addition od acid drives hydrogen cyanide equilibrium to the left and reduces the amount of nucleophiles.
- As a practical comprimise, the reaction between propanone and hydrogen cyanide is completed in solutions with an acidity no lower than pH 4.
Describe a test for aldehydes and ketones, creating a colour change.
Brady's reagent is an orange transparent mixture of methanol, sulfuric acid and a solution of 2,4-dinitrophenylhdrazine (2,4-DNP). When this is added to an aldehyde or ketone a yellow/orange precipitate of 2,4-dinitrophenylhdrazone derivative is seen. No precipitation is observed with a carboxylic acid or ester, despite these compounds also having the C=O bond.
What does qualitative analysis mean?
Qualitative analysis means using experiments to determine the presence or absence of a particular species. All the techniques described in this topic provide qualitative analysis as they allow the identification of a particular functional group.
After a positive Bradys reagent test, further processing of the precipitate allows the specific aldehyde or ketone to be identified. How can the specific aldehyde or ketone be identified?
- The 2,4-dinitrophenylhyfrazone derivative precipitate can be collected by filtration and purified using recrystallisation. After drying, the accurate melting point of the pure product ca then be measured through experiment.
- The aldehyde or ketone can then be identified by comparing the melting point the 2,4-dinitrophenylhydrazone derivatives precipitate with a database.
- The database is a list of accurately measured melting points of 2,4-dinitrophenylhydrazone derivatives listed against the aldehyde or ketone that made it.
When testing for specific ketones why is it necessary to test with Bradys reagent =, filtrate and recrystallise before testing the melting point?
Although every pure chemical has a specific melting and boiling point that would allow identification, this is experimentally difficult for similar ketones. Ketones with a similar chain length have very similar boiling points, making it challenging to experimentally distinguish between them. However, the 2,4-dinitrophenylhydrazone derivatives have very different melting points.
Apart from Brady's reagent, what else tests for aldehydes and ketones?
Tollens reagent also called ammoniacal silver nitrate.
How is the Tollens reagent test performed?
Tollens reagent also called ammoniacal silver nitrate, is a colourless chemical that is made in a two-stage process:
- Sodium hydroxide solution is added to silver nitrate solution until a brown precipitate is formed.
- Dilute ammonia is added drop-wise until the brown precipitate redissolves.
How does Tollens reagent distinguish between aldehydes and ketones?
- Tollens reagent can be used to distinguish between an aldehyde and a ketone. It is a weak oxidising agent and can react with the carbonyl functional group in an aldehyde but not a ketone. When Tollens' reagent is added to a ketone, there is no reaction. This is because ketones can not be oxidised further.
- When Tollens reagent is added to an aldehyde a silver mirror is observed.
- When aldehydes react with Tollens' reagent a redox reaction occurs. The silver ions are reduced and the aldehyde functional group is oxidised.
What is a balanced ionic equation for the oxidation of the silver ions in Tollens' reagent reacting with aldehydes?
Ag+(aq) + e- → Ag(s)
This causes silver metal to be precipitated out and this is observed as a silver mirror effect on the inside of the reaction vessel.
When Tollens' reagent is added to an aldehyde what happens to the aldehyde functional group?
The aldehyde functional group is oxidised to a carboxylic acid. In this equation, the oxidising agent is summarised as [O].
Define a base.
A base is a chemical that will react with an acid.
In the IUPAC naming convention, what takes the highest priority?
An acid takes the highest priority. So, the acid functional group generates the suffix of the name.
Describe the solubility of acids.
Small carboxylic acids are very soluble in polar solvents, this is because hydrogen bonds can be formed between the carboxylic acid functional group and water.
As the hydrocarbon chain of a carboxylic acid increases the solubility decreases (only the polar functional group can form hydrogen bonds).
What are the chemical properties of carboxylic acids?
Carboxylic acids are weak acids as they partially ionise in solution releasing the H+ ion from the carboxylic acid group, forming a carboxylate ion. So, this class of compounds will undergo typical acid reactions.
Carboxylic acid reactions will happen at a slower rate than with a strong acid as the pH is higher and therefore the concentration of H+(aq) will be lower.
Describe the reactions of carboxylic acids and metals.
- Carboxylic acids will react with metals above hydrogen in the reactivity series to make hydrogen and a metal salt.
- The name of the salt is generated from the acid.
- The suffix of the acid changes from -oic acid to -oate.
Give the word and symbol equation for sodium and ethanoic acid reacting.
sodium + ethanoic acid → sodium ethanoate + hydrogen
2Na + 2CH3COOH → 2CH3COONa + H2
Describe the reaction between metal oxides and carboxylic acids.
Metal oxides react with acids and therefore can be classified as bases. Carboxylic acids will react with metal oxides to make water and a metal salt.
Give the symbol and word equation for the reaction between magnesium oxide and methanoic acid.
magnesium oxide + methanoic acid → magnesium methanoate + water
MgO(s) + 2HCOOH(aq) → (HCOO)2Mg(aq) + H2O(l)
Describe the reaction between carboxylic acids and metal hydroxides.
Group 1 metal hydroxides are soluble bases that release OH-(aq) . Carboxylic acids will react with metal hydroxides to make water and a metal salt.
Give the word and symbol equation for the reaction of potassium hydroxide and propanoic acid.
Potassium hydroxide + propanoic acid → potassium propanoate + water
KOH(aq) + CH3CH2COOH(l) → CH3CH2COOK(aq) + H2O(l)
Describe the reaction of metal carbonates with carboxylic acids
Metal carbonates are also bases. Carboxylic acids will react with metal carbonates to make water, carbon dioxide and a metal salt.
Give the word and symbol equation between sodium carbonate and methanoic acid.
sodium carbonate + methanoic acid → sodium methanoate + water + carbon dioxide
Na2CO3 + 2HCOOH → 2HCOONa + H2O + CO2
How may carboxylic acids react with group 1 metal carbonates?
Group 1 metals can also for metal hydrogencarbonates. This is where the carbonic acid (H2CO3) has had only one proton exchanged for a metal ion to form the metal hydrogenarbonate (MHCO3 where M is a group 1 metal). Acids will also react with this class of compounds to form a salt, water and carbon dioxide.
What functional group do esters contain?
Esters contain the functional group R-COO-R' where R and R' are alkyl groups that may be the same or different.
How do you name an ester?
To name an ester, look at the structure of the ester - particulay the ester functional group. The first part of the name i the alkyl chain with a carbon atom bonded to only one oxygen atom. The second part of the name is the alkyl chain containing the C=O carbon atom followed by the suffix -oate.
What is esterification?
The chemical reaction used to make an ester is known as esterification. There are two main methods for making esters, carboxylic acids with an alcohol and acid anhydride with alcohol.
How are esters made with carboxylic acids?
- To prepare a small ester, an alcohol and a carboxylic acid are heated gently in the presence of a sulphuric acid catalyst.
- Esterification is a reversible reaction and has a slow rate of reaction.
What is the structural formula chemical equation for an esterification reaction to make ethyl ethanoate, using a carboxylic acid)?
In the reaction of carboxylic acids and an alcohol, how is the product separated from the reaction mixture?
- As the ester is volatile, with the lowest boiling point of the chemicals, it can be separated from the reaction mixture using distillation. The separation has to happen quickly to present the reverse reaction occurring.
- To prepare larger esters, the reaction mixture will need to be heated under reflux until the equilibrium has been established. The ester can be separated using fractional distillation.
When can the preparation of esters not use the reaction between carboxylic acids and an alcohol?
This method of preparation of an ester is not suitible for phenol or its derivatives as the rate of reaction is slow.
Describe the reaction of an acid anhydride to make an ester.
An acid anhydride is an acid derivative that is more reactive than a similar carboxylic acid. It is made by the removal of a molecule of water from two carboxylic acid molecules.
Acid anhydride will react wth alcohols, including phenol and its derivatives, to make an ester. This method of ester production is not reversible and therefore has a higher yield than using a carboxylic acid. The rate of reaction is still slow but can be increased by gently warming the reaction mixture.
Draw the displayed equation for the reaction between ethanoic anhydride and methanol
How is an acid anhydride formed?
An acid anhydride is formed by removal of a molecule of water from two carboxylic acid molecules.
Hydrolysis is a chemical reaction where water causes the breaking of bonds, in a decomposition reaction.
(esters → alcohol)
By varying the conditions, either carboxylic acids or carboxylate salts are formed as the other product.
Describe the hydrolysis of esters under acidic conditions.
When esters are refluxed with a catalyst of hot aqeous acids, such as dilute sulphuic acid or dilute HCl, the ester will decompose reversibly into an alcohol and a carboxylic acid.
Give the structural formula chemical reaction for the acid hydrolysis of propyl ethanoate.
Describe the hydrolysis of an ester in alkaline conditions.
- Alkaline chemicals are bases (react with acids) that can dissolve in water. When an ester is refluxed with hot aqueous alkalies, such as potassium hydroxide or sodium hydroxide, it will decompose into an alcohol and a carboxylate salt. This reaction is not reversible.
- Alkaline hydrolysis of esters is used to make soaps, therefore it is called saponification.
Draw the structural formula chemical equation for the alkaline hydrolysis of ethyl propanoate.
What functional group do acyl chlorides contain?
Acyl chlorides contain the functional group R-COCl where R is an alkyl group
What are acetyl chlorides derivatives of?
Carboxylic acids where the -OH of the acid group has been replaced with a chlorine atom.
Describe the properties of small acetyl chlorides.
Small acetyl chlorides are fuming (a visual gas or vapour being released) colourless liquids. They are very reactive, with the chlorine atom being substituted or other groups.
How do you name actyl chlorides?
Look at the structure of the compound and count the number of carbon atoms to generate the root. The suffix is -oyl chloride.
Describe the preparation of acyl chlorides.
To make an acyl chloride, the -OH group on a carboxylic acid must be substituted for a chlorine atom. One method is to use SOCl2, which is a liquid at room temperature ad readily reacts with a carboxylic acid to make the desired product. Sulphur dioxide and hydrogen chloride gases are also made. The acyl chloride is separated from the reaction mixture using distillation. The balanced chemical reaction shows the formation of ethanoyl chloride.
What is the balanced symbol equation for the formation of ethanoyl chloride?
CH3COOH + SOCl2 → CH3COCl + SO2 + HCl
Describe the uses of acyl chlorides as reagents in organic synthesis to produce esters from alcohols.
Acyl chlorides will react with alcohols to make an ester. This method of ester production is not reversible and therefore has a higher yield than using a carboxylic acid. The balanced chemical reaction shows the formation of an ester - ethyl ethanoate from an acyl chloride.
CH3COCl + CH3CH2OH → CH3COOCH2CH3 + HCl
Describe the uses of acyl chlorides as reagents in organic synthesis to produce esters.
Alcohols can also react with carboxylic acids to make esters. Like alcohols, phenols have an -OH group but they do not react easily with carboxylic acids. Therefore to make an ester from phenols the acyl chloride method must be used. However, the reaction is also violent and produces corrosive fumes of HCl.
Describe the uses of acyl chlorides as reagents in organic synthesis to produce carboxylic acids.
When a small acyl chloride such as ethanol is added to water, it quickly hydrolyses to produce a carboxylic acid. This is a very exothermic reaction and misty fumes of HCl are given off.
CH3COCl + H2O → CH3COOH + HCl
Describe the uses of acyl chlorides as reagents in organic synthesis to produce primary amides.
When an acyl chloride reacts with ammonia, a primary amide is produced. To prepare ethanamide, ethanoyl chloride is added to a concentrated ammonia solution. This quickly produces a mixture of solid ammonium chloride and ethanamide - observed as white smoke. Some of the products remain in a colourless solution.
CH3COCl + 2NH3 → CH3CONH2 + NH4Cl
Describe the uses of acyl chlorides as reagents in organic synthesis to produce secondary amides.
When an acyl chloride reacts with a primary amide, the product is a secondary amide, where the nitrogen has one hydrogen atom directly bonded to it. The nitrogen atom also has two organic groups attached and if often called an N-substituted amide.
A white solid compound of N-ethylethanamide can be made from ethanol chloride and a cold concentrated solid of ethylamine.
CH3COCl + CH3CH2NH2 → CH3CONHCH2CH3 + HCl
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