Types of Reactions Part 2.pdf

Addition Reactions Organic chemistry Addition Reactions 6- Halohydrin formation Addition of chlorine or bromine in the presence of water can yield compounds containing halogen and hydroxyl groups on adjacent carbon atoms. These compounds are commonly referred to as halohydrins. Addition Reactions 6-...

Addition Reactions Organic chemistry Addition Reactions 6- Halohydrin formation Addition of chlorine or bromine in the presence of water can yield compounds containing halogen and hydroxyl groups on adjacent carbon atoms. These compounds are commonly referred to as halohydrins. Addition Reactions 6- Halohydrin formation Propylene gives the chlorohydrin in which chlorine is attached to the terminal carbon. Orientation follows Markovnikov's rule, with positive halogen going to the same carbon that the hydrogen of a protic reagent would. Addition Reactions 7- Addition of alkenes. Dimerization Under proper conditions, isobutylene is converted by sulfuric or phosphoric acid into a mixture of two alkenes of molecular formula C8H16. The two alkenes are isomers, and differ only in position of the double bond. Addition Reactions 7- Addition of alkenes. Dimerization Since the alkenes produced contain exactly twice the number of carbon and hydrogen atoms as the original isobutylene. They are known as dimers (di = two, mer = part) of isobutylene, and the reaction is called dimerization. Addition Reactions 8- Addition of alkanes. Alkylation Isobutylene (alkene) and isobutane (alkane) are allowed to react in the presence of an acidic catalyst, to form directly 2,2,4-trimethylpentane. Addition Reactions 9- Hydroboration-oxidation With the reagent diborane, (BH3)2, alkenes undergo hydroboration to yield alkylboranes, R3B, which on oxidation give alcohols. Addition Reactions 10- Free-radical polymerization of alkenes When ethylene is heated under pressure with oxygen, there is obtained a compound of high molecular weight (about 20,000), which is essentially an alkane with a very long chain. This compound is made up of many ethylene units and hence is called polyethylene (poly = many). Addition Reactions 10- Free-radical polymerization of alkenes The formation of polyethylene is a simple example of the process called polymerization: the joining together of many small molecules to make very large molecules. The compound composed of these very large molecules is called a polymer. The simple compounds from which polymers are made are called monomers (mono= one). Polymerization of substituted ethylenes yields compounds whose structures contain the long chain of polyethylene, with substituents attached at more or less regular intervals. Addition Reactions 10- Free-radical polymerization of alkenes Polymerization requires the presence of a small amount of an initiator. Among the commonest of these initiators are peroxides, which function by breaking down to form a free radical. This radical adds to a molecule of alkene, and in doing so generates another free radical. This radical adds to another molecule of alkene to generate a still larger radical, which in turn adds to another molecule of alkene, and so on. Addition Reactions 10- Free-radical polymerization of alkenes Eventually the chain is terminated by steps, such as union of two radicals that consume but do not generate radicals. This kind of polymerization, each step of which consumes a reactive particle and produces another, similar particle, is an example of chain-reaction polymerization. Addition Reactions 11- Hydroxylation. Glycol formation Certain oxidizing agents convert alkenes into compounds known as glycols. Glycols are simply dihydroxy alcohols; their formation amounts to the addition of two hydroxyl groups to the double bond. Of the numerous oxidizing agents that cause hydroxylation, two of the most commonly used are cold alkaline potassium permanganate KMnO4, and peroxyformic acid HCO2OH Addition Reactions 12- Substitution Reactions: Halogenations. Allylic substitution. We know that alkenes undergo addition of halogen at low temperatures and in the absence of light, and generally in the liquid phase: Conditions that favor ionic reactions, or at least do not aid formation of radicals. If we wish to direct the attack of halogen to the alkyl portion of an alkene molecule, then, we choose conditions that are favorable for the freeradical reaction and unfavorable for the ionic reaction, at a temperature of 500-600 oC Addition Reactions 12- Substitution Reactions: Halogenations. Allylic substitution. A mixture of gaseous propylene and chlorine yields chiefly the substitution product, 3-chloro-l-propene, known as allyl chloride (CH2=CH─CH2─ = allyl). Bromine behaves similarly. Addition Reactions 13- Ozonolysis Along with addition and substitution we may consider a third general kind of alkene reaction, cleavage: a reaction in which the double bond is completely broken and the alkene molecule converted into two smaller molecules. The classical reagent for cleaving the carbon-carbon double bond is ozone. Ozonolysis (cleavage "by ozone) is carried out in two stages: First, addition of ozone to the double bond to form an ozonide; Second, hydrolysis of the ozonide to yield the cleavage products. Addition Reactions 13- Ozonolysis These compounds containing the C═O group are called aldehydes and ketones. Alicyclic Hydrocarbons The Alicyclic Hydrocarbons (Aliphatic Cyclic Hydrocarbons) Alicyclic compound contains one or more all-carbon rings which may be either saturated or unsaturated, but do not have aromatic character. Alicyclic compounds may have one or more aliphatic side chains attached. Alicyclic Hydrocarbons The Alicyclic Hydrocarbons (Aliphatic Cyclic Hydrocarbons) The carbon atoms are attached to one another to form chains; these are called open-chain compounds. In many compounds, however, the carbon atoms are arranged to form rings; these are called cyclic compounds. Naming Cycloalkane Naming Cycloalkane Substituted cycloalkanes are named by rules similar to those for open-chain alkanes For most compounds, there are only two steps. Naming Cycloalkane Step 1: Find the parent. Count the number of carbon atoms in the ring and the number in the largest substituent. If the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane. If the number of carbon atoms in the largest substituent is greater than the number in the ring, the compound is named as a cycloalkyl-substituted alkane. For example: Naming Cycloalkane Step 2: Number the substituents, and write the name. For an alkyl- or halo-substituted cycloalkane, choose a point of attachment as carbon 1 and number the substituents on the ring so that the second substituent has as low a number as possible. If ambiguity still exists, number so that the third or fourth substituent has as low a number as possible, until a point of difference is found. Naming Cycloalkane Naming Cycloalkane Step 2: Number the substituents, and write the name. When two or more different alkyl groups are present that could potentially take the same numbers, number them by alphabetical priority. If halogens are present, treat them just like alkyl groups. Naming Cycloalkane Cycloalkenes are named similarly to alkene, but because there is no chain end to begin from, we number the cycloalkene so that the double bond is between C1 and C2 and the first substituent has as low a number as possible. It’s not necessary to indicate the position of the double bond in the name because it’s always between C1 and C2. Naming Cycloalkane Preparation Alicyclic hydrocarbons are prepared from other cyclic compounds (e.g., halides or alcohols) by exactly the same methods that are used for preparing open-chain hydrocarbons from other open-chain compounds. Naming Cycloalkane Reactions With certain very important and interesting exceptions, alicyclic hydrocarbons undergo the same reactions as their open-chain analogs. Cycloalkanes undergo chiefly free-radical substitution. For example: Naming Cycloalkane Cycloalkenes undergo chiefly addition reactions, both electrophilic and free radical; like other alkenes, they can also undergo cleavage and allylic substitution. For example: