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Interconversions in Organic Chemistry
Transcript of Interconversions in Organic Chemistry
Forming Alcohols from Halogenoalkanes-
The conversion of halogenoalkanes into alcohols involves the process of nucleophillic substitution. During the conversion, the halogenoalkane will be heated under
with aqueous sodium-hydroxide. A polar bond exists between the carbon atom and the bromine atom of the halogenoalkane. In this case, the hydroxide ion acts as the nucleophile, with the negatively charged oxygen atom being attracted to the delta positively charged carbon atom. The lone pair of electrons on the oxygen atom forms a covalent bond with the carbon atom, as the bond between the carbon and bromine atom breaks. The breaking of this bond involves the donation of the bonding pair of electrons (in which one belonged originally to the carbon atom), to the bromine atom, so as to form a bromide ion. The eventual result is the formation of an Alcohol and the bromide ion. This reaction involves hetrolytic fission, as ions, as opposed to free radicals, are formed. No carbocation is formed at any point during this process, as the breaking and formation of new bonds is continuous and fluid.
Forming Halogenoalkanes from Alcohols-
To reverse the conversion of halogenoalkanes into alcohols, nucleophilic substitution is once again employed, with bromide or chloride ions acting as the nucleophiles in this case. The alcohol will be reacted with aqueous hydrochloric or hydrobromic acid, dependant on the Halogenoalkane that is to be formed. Hydrobromic acid is prepared whilst in the reaction mixture, via the independent reaction of sodium-bromide with sulphuric acid. The formation of a bromoalkane will occur under
. The alcohol in question must be initially protonated in a strongly acidic solution- a process which involves the bonding of a hydorgen ion to the oxygen atom of the alcohol. Said addition allows for the carbon atom, which is bonded to the oxygen atom, to achieve a greater delta positive charge and subsequently become more open to 'attack' by the nucleophiles. Typical nucleophilic substitution will follow, with the nucleophile attacking the delta positive carbon atom so as to form a covalent bond. The introduction of new electrons removes the original side group, successfully forming a halogenoalkane and water.
While common, it is not essential for nucleophiles to be fully negatively charged. Water molecules, for example, are neutral, yet may act as nucleophiles. This is due to the presence of the lone pairs on the oxygen atom. One of these lone pairs is able to form a covalent bond with the carbon atom of the halogenoalkane. The bond between the carbon and bromine atom once again breaks to form the bromide ion. As is shown in the diagram, the resulting complex ion will then lose a hydrogen ion, to form an alcohol. This reaction is markedly slower than the
process in which sodium-hydroxide is a reagent and is known as a hydrolysis reaction.
Nucleophiles- Molecules or negatively charged ions with lone pairs which can be donated to a positively/delta positively charged atom, so as to form a covalent bond.
Electrophiles- Molecules or ions with a positive (or delta positive) charge which will attack electron rich areas- e.g. C-C double bonds. They are additionally attracted to polarised atoms with a subsequent delta negative charge. Electrophiles react by accepting a lone pair of electrons to form a covalent bond.
Free Radicals- Are atoms with unpaired singular or multiple electrons, which it desires to pair up. Free radicals are highly reactive but are uncharged.
Forming Amines from Halogenoalkanes-
There are a variety of different molecules that can act as nucleophiles and consequently result in the conversion of halogenoalkanes into a variety of different chemicals. Ammonia (NH3) is one such molecule, behaving in a similar fashion to water. The lone pair of the nitrogen atom will attack the delta positively charged carbon atom (which is connected to the halogen atom by a polarised bond) to produce an amine (with an NH2 group). The halogenoalkane must simply be heated in a sealed tube with a solution of ammonia. Amines themselves have lone pairs of electrons on their nitrogen atoms and can consequently behave as nucleophiles in their own right. Progressive substituion reactions involving amines as nucleophiles will result in the production of secondary and tertiary amines.
Forming Halogenoalkanes from Alkenes-
Electrophilic addition is key in the conversion of alkenes into halogenoalkanes. One such example of this process is the conversion of ethene into dibromoethane (and bromoethane additionally). During the reaction, as shown in the diagram below, a molecule of bromine (Br2) becomes polarised as it draws into close proximity with the alkene. The repulsive forces that exist between the electrons of the alkene double bond and those of the bond in the bromine molecule, result in the appearance of an induced dipole between the bromine atoms.
The subsequently delta positive bromine atom nearest to the ethene molecule, will behave as an electrophile- reacting to bond with the ethene molecule and leave behind a solitary bromide ion (which later in the process will act helpfully as a nucleophile). The consequent molecule formed by the bonding of the bromine atom will contain a newly positively charged carbon atom, as the atom will have access to only 6 surrounding electrons. The presence of such a charge earns this atom the title of a 'carbocation'- a phenomenon that does not occur in smooth, swift processes, such as nucleophillic substitution to form alcohols. This carbocation will be keen to react with any surrounding nucleophiles, including the remaining bromide ion . The transferral of a pair of electrons from the bromide ion to the charged carbon atom will conclude in the formation of a new covalent bond and the production of dibromethane. This reaction will occur in an organic solvent at room temperature.
Bromine water may be used as an alternative to pure concentrated bromine in this reaction. If such is the case, water molecules may compete with bromide ions for the role of the nucleophiles in the latter stage of the conversion. A more diluted sample of bromine water may result in the successful bonding of water molecules and the subsequent formation of a bromoalcohol.
Ethene will react equally with concentrated hydrogen bromide at room temperature to simply produce bromoethane.
Converting Alkenes into Alkanes-
A further instance in which an addition reaction will occur is the conversion of alkenes into alkanes. The reagents, in this case ethene and hydrogen molecules, will be absorbed onto the surface of the heterogenous catalyst and the bonds between the atoms of the molecules will be consequently weakened. The bonds between the hydorgen atoms will break, as will the double bond of the ethene molecule, to leave a single bond between the two carbon atoms. New covalent bonds will proceed to form between the carbon atoms of the ethene molecule and the independent hydrogen atoms, thus producing a molecule of ethane. A platinum or nickel catalyst may be used, however, while notably less expensive, the latter acts far less efficiently and must be ground into a fine powder before use. When using nickel the gaseous reagents must be heated to 150°C at a pressure of 5 atmospheres, whilst the reaction may simply occur under typical laboratory conditions when using the platinum catalyst. This reaction is referred to as hydrogenation.
Interconversions in Organic Chemistry
The Formation of Polymers-
Polymers are long chain molecules comprised of many individual smaller molecules, called monomers. Many of these polymers are formed during addition reactions. Addition polymerisation will typically use alkenes as the monomers. The double bonds of the alkene monomers will open up to form covalent bonds with the neighbouring monomers- which are ultimately compiled into a single chain. Co-polymerisation occurs when more than one type of monomer is used during the addition reaction. These monomers will join together in a random order, forming a polymer with properties that will differ from the original 'pure' polymers. Soft, spring polymers that will return to their shape following deformation are called elastomers, whilst slightly more rigid polymers that undergo permanent deformation are know as plastics. Other polymers may be spun into thin durable threads. These can then be used as fibres in clothing and other materials. Chain length, the presence of branching, the polarity and position of side groups and cross linking, will all affect the properties of the polymer in question. Cross linking refers to the presence of many covalent bonds between the polymer chains of a material. Those chains which do not have cross linking have comparatively much weaker intermolecular bonds and are therefore free to move over one another- creating a malleable material that, when heat is applied (to break the bonds), may be deformed and set into a new shape. 'Thermosetting' polymers have cross links between their chains. These strong covalent bonds hold the chains staunchly in place- preventing them from moving over each other and subsequently creating a far more rigid material. Such polymers will not melt or be easily deformed, as a greater amount of energy is required to overcome the strong intermolecular forces between the chains. Many thermosetting polymers will simply char and decompose when heated, as opposed to melting. Addition polymerisation will occur at a temperature of 200°C and at a pressure of 1500 atmospheres, using a gaseous trace of O2 during the process.
Forming aldehydes from alcohols-
The hyroxyl group of alcohols may be oxidised to form various other organic chemicals- namely Aldehydes and Ketones. During the oxidation process, two atoms of hydrogen are removed from a carbon atom and the oxygen atom of the hydroxyl group- leaving a carbonyl group. Oxidation of the alkyl group will not take place unless it is attached to a carbon atom that is, in turn, connected to a second hydrogen atom. Primary alcohols have the alkyl group positioned at the end of the hydrocarbon chain and will therefore be oxidised to become aldehydes. While aldehydes may be oxidised further to become carboxylic acids, this is not always desirable. To acheive an aldehyde, the product must be distilled off and heating ceased before it is converted further into a carboxylic acid.
Forming ketones from alcohols-
The hydroxyl group of secondary alcohols will be in the middle of the hydrocarbon chains, causing them to be oxidised to ketones. Tertiary alcohols have an alkyl group that is attached to a carbon with no further hydrogen atom connection, so they cannot be oxidised. Ketones are far less readily oxidised than aldehydes, wich require onlhy very weak oxidising agents such as Fehling's solution, which contains (Cu)2+ ions and alkali. Ketones are not oxidised by such
Oxidation reactions, such as those of alcohols, occur under reflux with a strong oxidising agent- such as acidified potassium dichromate. Heating under reflux with the correct apparatus will ensure that no reagents or product that is produced can escape.
The reagents should first be placed in a round bottomed/ pear shaped flask and a few anti-bumping granules added to ensure that the reaction mixture does not boil and result in the 'bumping' of the equipment. Be careful not to stopper the flask, as doing so may cause a build up in pressure that will crack the glassware/ initiate its explosion.
A condenser should then be attached to the flask vertically. The condenser will prevent vaporous products remaining as such, converting them back to liquids that will remain within the flask.
Using a Bunsen burner or, more preferably, a heating mantle (which brings with it a reduced safety risk), heat the reaction mixture. Continue to do so until the reaction has reached completion. In the case of the oxidation of alcohols, this will be indicated by a change in colour from orange to green, as the orange dichromate (VI) ion, (Cr2O7)2- is reduced to the green (Cr)3+ ion.
Aldehydes produced via the oxidation of primary alcohols may be further oxidised themselves to produce carboxylic acids. To ensure that a carboxylic acid is produced, simply continue to heat the alcohol reaction mixture well beyond the point of conversion into an aldehyde.
In order to separate the carboxylic acid you have produced from the other components of the reaction mixture, it is necessary to use simple distillation- a typical purification method.
The reaction mixture in the pear shaped flask, including the small number of anti-bumping granules. Arrange the apparatus as displayed in the diagram, making sure that a constant supply of cold water is provided into the condenser.
Using a Bunsen boiler or heating mantle heat the mixture once again. Continue to heat the mixture until all of the distilled contents have been collected in the collection beaker. Alternatively, if the boiling point of the carboxylic acid is known, insert a thermometer into the pear shaped/round bottomed flask, as shown in the diagram. When the temperature of the vapour rises above the boiling point of the collected liquid, heating may cease and it is safe to assume that a reasonable volume of the desired liquid will have been obtained.
If you wish to check that you have achieved a carboxylic acid, a simple test may be performed using universal indicator.
Converting Alkenes into Alcohols-
A similar reaction, this time using water as the polarisable molecule, so as to produce an electrophile in the form of a hydrogen ion (and use the subsequent remaining hydroxide ion as a nucelophile to complete the addition reaction), can be used to convert an alkene into an alcohol. This reaction will occur in the presence of a phosphoric acid absorbed onto solid silica catalyst, under conditions of high temperature and pressure. In the laboratory, concentrated sulphuric acid may be added to an alkene (such as ethene), the molecules of which will become polarised to allow for the elctrophilic addition of hydrogen to the ethene molecule. The subsequent carbocation will bond to the remaining negatively charged OSO3H, forming a molecule that can then be diluted with water. The water molecules will behave as nucleophiles within a successive nucleophilic substitution, to form ethanol and sulphuric acid. This is an example of a hydration reaction. An industrial alternative to this process is the action of a steam and phosphoric acid catalyst. This occurs under pressure 60 atmospheres and temperature 300°C and results in successful hydration.
Coverting Alcohols into Alkenes-
During an elimination reaction, a small molecule is removed from a larger molecule to result in the original molecule becoming unsaturated. For alcohols, this elimination takes the form of dehydration, during which a molecule of water is lost to produce an alkene. When propan-1-ol is maintained under high temperature conditions of 300°C and interacts with a heated aluminia catalyst (Al2O3), a molecule of water will be lost to form propene. A slightly differing method of dehydration involves the heating of the alcohol with concentrated sulphuric acid, performed under
Converting Alcohols into Ethers-
Primary alcohols may be converted into ethers during a condensation reaction, in which two molecules will interact and combine to produce a larger molecule, whilst releasing a smaller molecule in conjunction with the first. The alcohol will be heated alongside a sulphuric acid catalyst and two molecules of the alcohol will react to create an ether and release water. With regards to the mechanism of the reaction, a hydrogen ion is lost from the sulphuric acid catalyst and, as an electrophile, bonds to the ethanol molecule to produce an ethyloxonium ion and a hydrogen sulphate ion.
During a process of nulceophilic substitution, a secondary molecule of ethanol will bond to the ethyloxonium ion, removing a molecule of water and creating a diethyloxonium ion.
The hydrogen atom attached to the oxygen atom of the diethyloxonium ion is lost as a hydrogen ion. The molecule is said to have been 'deprotonated'. The hydrogen ion behaves as an electrophile- participating in electrophilic addition, so as to bond to a further molecule of ethanol. This final stage sees the production of the ether (diethyl ether) and the side release of a ethyloxonium ion.
Esters are formed in a condensation reaction between an alcohol and a carboxylic acid. The reagents will be heated together, under
, in the presence of a concentrated acid catalyst (either hydrochloric or sulphuric acid). The process by which an ester bond/link is formed between the hydroxyl (OH) group of the alcohol and the carboxyl (COOH) group of the carboxylic acid, is called esterification. The production of esters is a reversible reaction, with the backwards process know as ester hydrolysis. As if, according to Le Chatelier's Principle, the conditions of a dynamic equilibrium are changed, the position of the equilibrium will move to act against this change, adding a greater volume of alcohol will shift the position of the equilibrium to the right and result in a greater yield of ester.
Esters are not formed exclusively via the reaction of an alcohol and carboxylic acid. Phenol, an aromatic compound with an hydroxyl group, may undergo esterification. However, the hydroxyl group of phenol is far less reactive than those of alcohols and so requires a reaction with something somewhat more vigorous. The ethanoylating agent ethanoic anhydride (an acid anhydride) may be react with salicylic acid (an aromatic compound) to produce aspirin. This reaction will take place under reflux.
Another, far more reactive and equally more toxic, ethanolyating agent, is ethanoyl chloride (an acyl chloride). The ethanoyl chloride will react with the hydroxyl group of phenol to produce phenyl ethanoate. This reaction does not occur under reflux but may simply happen at room temperature.
Esters may be converted back into the original alcohol and carboxylic acid through a hydrolysis reaction, in which water is added to the ester bond. A catalyst will commonly be used to increase the rate of reaction, most often dilute sulphuric acid. Once again with reference to Le Chatelier's Principle, greatly increasing the volume of water will shift the position of the equilibrium to the left, resulting in an increase in the yield of alcohol and carboxylic acid product.
Fehling's solution can be used to differentiate between aldheydes and ketones. While ketones are not readily oxidised by the solution, aldehydes are. As the aldehyde is oxidised, the blue (Cu)2+ of the solution are reduced to Cu+ ions. From the reaction mixture a solid orange precipitate of Cu2O is formed- indicating the presence of an aldehyde and the completion of the oxidation process.
The reduction of an aldehyde using sodium-tetrahydridoborate as the reducing agent.
The reduction of a ketone using sodium-tetrahydridoborate. This complex metal anhydride is necessary as neither aldheydes or ketones will oxidise easily.
Both aldheydes and ketones are able to undergo addition reactions so as to produce new compounds. A key and interesting example of such is the formation of cyanohydrins. Aldehydes and ketones may both be reacted with hydrogen cyanide under alkali conditions to produce a cyanohydrin during a process of nucleophilic addition. The cyanide ion in the hydrigen cyanide solution will behave as the nucleophile, attacking the delta positive carbon atom, which carries such a charge due to the large dipole which exists between said atom and the oxygen atom to which it is connected. A covalent bond forms between the cyanide group and the central carbon atom, while the consequential distribution of electrons results in a negative charge on the oxygen atom. As the reaction will occur in solution, a hydrogen ion from the surrounding solvent, water, will behave as an electrophile- attacking the oxygen ion to form a cyanohydrin in a final stage of electrophilic addition. Despite this latter process, the reaction overall is referred to as a nucleophilic addition reaction.
Acylchlorides may be reacted with amines to form amides. This reaction is referred to as an acyltation- the adding of an acyl group to the amine group. The diagram shows the acyltation of ammonia to form the primary amide ethanamide. The lone pair on the ammonia molecule attacks the central delta positive carbon atom- made positive by the dipole that exists between said atom and the oxygen atom connected to it. This results in the bonding of the ammonia to the central carbon and the donation of an electron pair from the C-O bond to the oxygen atom, consequently forming a negative oxygen ion. In a continuous sequence, the electron pair on the oxygen atom will return to restore the double bonded system, whilst an electron pair is donated to the chlorine atom (forming an independent chlroide ion) and hydrogen performs the reverse to form a hydrogen ion. This ultimately produces the amide ethanamide and HCl
An alkali such as sodium-hydroxide will be a sufficient substitute for water in the breakdown of esters. Instead of a carboxylic acid a carboxylate salt e.g. soiumcarboxylate, will be produced. In this situation the reagents will be heated under