CH 5: An Overview of Organic Reactions Vanessa N. Prasad-Permaul CHM 1046 Valencia College

Why this chapter? To understand organic and/or biochemistry, it is necessary to know: -What occurs -Why and how chemical reactions take place We will see how a reaction can be described

HYDROCARBONS SATURATED HYDROCARBONS: hydrocarbons that contain only single bonds between carbon atoms UNSATURATED HYDROCARBONS: hydrocarbons that contain double or triple bonds between carbon atoms AROMTAIC HYDROCARBONS: hydrocarbons that contain benzene rings or similar features

The Bonding of Carbon C Alkane Single Bond Double Bond Alkene Two Double Bond Triple Bond Alkyne

Alkanes Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups) Connecting carbons can lead to large or small molecules The formula for an alkane with no rings in it must be CnH2n+2 where the number of C’s is n Alkanes are saturated with hydrogen (no more can be added They are also called aliphatic compounds

Structures of Alkanes

Branched-Chain Alkanes Isomers: compounds with the same molecular formula but different structural formulas

Branched-Chain Alkanes

EXAMPLE 23.1 EXERCISE 23.1

Kinds of Organic Reactions In general, we look at what occurs and try to learn how it happens Common patterns describe the changes Addition reactions – two molecules combine Elimination reactions – one molecule splits into two Substitution – parts from two molecules exchange Rearrangement reactions – a molecule undergoes changes in the way its atoms are connected

SUBSTITUTION REACTIONS OF ALKANES

ALKENES GEOMETRIC ISOMERS: isomers in which the atoms are joined to one another in the way but differ because some atoms occupy different relative positions in space

EXAMPLE 23.2 EXERCISE 23.2

REARRANGEMENT OF ALKENES

ADDITION OF ALKENES Addition

MARKOWNIKOFF’S RULE: "when an unsymmetrical alkene reacts with a hydrogen halide to give an alkyl halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents yielding the major product" 2-bromopropane 1-bromopropane

EXAMPLE 23.3 EXERCISE 23.3

ALKYNES Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature. 1-butyne 2-chloro-1-butene 2,2 dichloro butane

AROMATIC HYDROCARBONS

SUBSTITUTION REACTIONS OF AROMATIC HYDROCARBONS

NOMENCLATURE OF ALKANES The parent name of the molecule is determined by the number of carbons in the longest chain. In the case where two chains have the same number of carbons, the parent is the chain with the most substituents. The carbons in the chain are numbered starting from the end nearest the first substituent. In the case where there are substituents having the same number of carbons from both ends, numbering starts from the end nearest the next substituent. When more than one of a given substituent is present, a prefix is applied to indicate the number of substituents. Use di- for two, tri- for three, tetra- for four, etc. and use the number assigned to the carbon to indicate the position of each substituent. 3-METHYLHEPTANE

Branched Alkanes Branched substituents are numbered starting from the carbon of the substituent attached to the parent chain. From this carbon, count the number of carbons in the longest chain of the substituent. The substituent is named as an alkyl group based on the number of carbons in this chain. Numbering of the substituent chain starts from the carbon attached to the parent chain. The entire name of the branched substituent is placed in parentheses, preceded by a number indicating which parent-chain carbon it joins. Substituents are listed in alphabetical order. To alphabetize, ignore numerical (di-, tri-, tetra-) prefixes (e.g., ethyl would come before dimethyl), but don't ignore don't ignore positional prefixes such as iso and tert (e.g., triethyl comes before tertbutyl).

EXAMPLE 23.4 EXERCISE 23.4

EXAMPLE 23.5 EXERCISE 23.5

NOMENCLATURE OF ALKENES Alkenes are named by dropping the -ane ending of the parent and adding -ene. The parent structure is the longest chain containing both carbon atoms of the double bond Give the double bond the lowest possible numbers regardless of substituent placement. GIVE THE STRUCTURAL FORMULA AND NAME THE FOLLOWING COMPOUND: 4-methyl-2-pentene

EXERCISE 23.6 EXERCISE 23.7 EXERCISE 23.8

NOMENCLATURE OF ALKYNES Alkynes are named by dropping the -ane ending of the parent and adding -yne. The parent structure is the longest chain containing both carbon atoms of the double bond Give the double bond the lowest possible numbers regardless of substituent placement. EXAMPLES: EXERCISE 23.9

NOMENCLATURE OF AROMATIC HYDROCARBONS Benzene is the most common aromatic parent structure. Multiple substituents on a benzene ring are numbered to give these substituents the lowest possible numbers. When only two substituents are attached to a benzene ring, they can be named by the common nomenclature using ortho (o-) (1-2 placement), meta (m-) (1-3 placement) or para (p-) (1-4 placement).

EXERCISE 23.10 EXAMPLES 1,2,4-trimethylbenzene p-dimethylbenzene 2,3'-dimethylbiphenyl EXERCISE 23.10

Functional Groups of Organic compounds

ELIMINATION OF ALCOHOLS

NOMENCLATURE OF ALCOHOLS PRIMARY ALCOHOL SECONDARY ALCOHOL TERTIARY ALCOHOL

Skeletal formulae EXERCISE 23.11 In a skeletal formula, all the hydrogen atoms are removed from carbon chains, leaving just a carbon skeleton with functional groups attached to it. For example; 2-butanol. The normal structural formula and the skeletal formula look like this: EXERCISE 23.11

NOMENCLATURE OF ETHERS Ethers can be named by naming each of the two carbon groups as a separate word followed by a space and the word ether. The –OR group can also be named as a substituent using the group name, alkoxy. CH3–CH2–O–CH3 ethyl methyl ether methoxyethane cyclopentyl methyl ether methoxycyclopentane

EXAMPLES: 2-pentyl 1-propyl ether EXERCISE 23.12

NOMENCLATURE OF ALDEHYDES AND KETONES Aldehydes and ketones are organic compounds which incorporate a carbonyl functional group, C=O. The carbon atom of this group has two remaining bonds that may be occupied by hydrogen or alkyl or aryl substituents. If at least one of these substituents is hydrogen, the compound is an aldehyde. If neither is hydrogen, the compound is a ketone.

The IUPAC system of nomenclature assigns a characteristic suffix to these classes, al to aldehydes and one to ketones

EXAMPLES: EXERCISE 23.13

NOMENCLATURE OF CARBOXYLIC ACIDS As with aldehydes, the carboxyl group must be located at the end of a carbon chain. In the IUPAC system of nomenclature the carboxyl carbon is designated #1, and other substituents are located and named accordingly. The characteristic IUPAC suffix for a carboxyl group is "oic acid", and care must be taken not to confuse this systematic nomenclature with the similar common system.

Salicylic Acid (2-hydroxybenzoic acid) EXERCISE 23.14

NOMENCLATURE OF ESTERS To name an ester you treat it as a derivative of an alcohol and an acid, which it is. First you name the part from the alcohol and then the part from the acid using an -ate ending. The resulting name should be written as two words. This diagram shows an ester that is made from methyl alcohol and acetic acid. So its name is methyl acetate. Methyl from the methyl alcohol. Acetate from the acetic acid.

NOMENCLATURE OF ESTERS Esters are named as derivatives of the carboxylic acid from which they are formed.   Condensation of ethanoic acid with methanol will produce methyl ethanoate.   As stated above the ending of the acid -oic is changed to -oate, much as if the ester were a salt of the acid.  The esterification reactions are generally easily reversible by addition of water; the reverse reaction is called the hydrolysis of the ester and proceeds in the presence of aqueous base. Methyl ethanoate CH3OH + CH3COOH CH3COOCH3 + H2O methanol  + ethanoic acid methyl ethanoate + water

EXERCISE 23.15

How Organic Reactions Occur: Mechanisms In an organic reaction, we see the transformation that has occurred. The mechanism describes the steps behind the changes that we can observe Reactions occur in defined steps that lead from reactant to product

Steps in Mechanisms We classify the types of steps in a sequence A step involves either the formation or breaking of a covalent bond Steps can occur in individually or in combination with other steps When several steps occur at the same time they are said to be concerted

Types of Steps in Reaction Mechanisms Bond formation or breakage can be symmetrical or unsymetrical Symmetrical- homolytic Unsymmetrical- heterolytic

Indicating Steps in Mechanisms Curved arrows indicate breaking and forming of bonds Arrowheads with a “half” head (“fish- hook”) indicate homolytic and homogenic steps (called ‘radical processes’) Arrowheads with a complete head indicate heterolytic and heterogenic steps (called ‘polar processes’)

Radical Reactions Not as common as polar reactions Radicals react to complete electron octet of valence shell A radical can break a bond in another molecule and abstract a partner with an electron, giving substitution in the original molecule A radical can add to an alkene to give a new radical, causing an addition reaction

Steps in Radical Substitution Three types of steps Initiation – homolytic formation of two reactive species with unpaired electrons Example – formation of Cl atoms form Cl2 and light

Steps in Radical Substitution Propagation – reaction with molecule to generate radical Example - reaction of chlorine atom with methane to give HCl and CH3.

Steps in Radical Substitution Termination – combination of two radicals to form a stable product: CH3. + CH3.  CH3CH3

An Example of a Polar Reaction: Addition of HBr to Ethylene HBr adds to the  part of C-C double bond The  bond is electron-rich, allowing it to function as a nucleophile H-Br is electron deficient at the H since Br is much more electronegative, making HBr an electrophile

Mechanism of Addition of HBr to Ethylene HBr electrophile is attacked by  electrons of ethylene (nucleophile) to form a carbocation intermediate and bromide ion Bromide adds to the positive center of the carbocation, which is an electrophile, forming a C-Br  bond The result is that ethylene and HBr combine to form bromoethane All polar reactions occur by combination of an electron-rich site of a nucleophile and an electron-deficient site of an electrophile

Using Curved Arrows in Polar Reaction Mechanisms Curved arrows are a way to keep track of changes in bonding in polar reaction The arrows track “electron movement” Electrons always move in pairs Charges change during the reaction One curved arrow corresponds to one step in a reaction mechanism

Rules for Using Curved Arrows The arrow goes from the nucleophilic reaction site to the electrophilic reaction site The nucleophilic site can be neutral or negatively charged

Rules for Using Curved Arrows The electrophilic site can be neutral or positively charged The octet rule must be followed

Describing a Reaction: Energy Diagrams and Transition States The highest energy point in a reaction step is called the transition state The energy needed to go from reactant to transition state is the activation energy (DG‡)

First Step in Addition In the addition of HBr the (conceptual) transition-state structure for the first step The  bond between carbons begins to break The C–H bond begins to form The H–Br bond begins to break

Describing a Reaction: Intermediates If a reaction occurs in more than one step, it must involve species that are neither the reactant nor the final product These are called reaction intermediates or simply “intermediates” Each step has its own free energy of activation The complete diagram for the reaction shows the free energy changes associated with an intermediate

A Comparison between Biological Reactions and Laboratory Reactions Laboratory reactions usually carried out in organic solvent Biological reactions in aqueous medium inside cells They are promoted by catalysts that lower the activation barrier The catalysts are usually proteins, called enzymes Enzymes provide an alternative mechanism that is compatible with the conditions of life