Chapter 7 - Organic chemistry - Edrolo - Textbook PDF v1 PDF

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Assisted in understanding Organic Chem...

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UNIT

4

How are organic compounds categorised, analysed and used?

The carbon atom has unique characteristics that explain the diversity and number of organic compounds that not only constitute living tissues but are also found in the fuels, foods, medicines and many of the materials we use in everyday life. In this unit students investigate the structural features, bonding, typical reactions and uses of the major families of organic compounds including those found in food. Students study the ways in which organic structures are represented and named. They process data from instrumental analyses of organic compounds to confirm or deduce organic structures, and perform volumetric analyses to determine the concentrations of organic chemicals in mixtures. Students consider the nature of the reactions involved to predict the

products of reaction pathways and to design pathways to produce particular compounds from given starting materials. Students investigate key food molecules through an exploration of their chemical structures, the hydrolytic reactions in which they are broken down and the condensation reactions in which they are rebuilt to form new molecules. In this context the role of enzymes and coenzymes in facilitating chemical reactions is explored. Students use calorimetry as an investigative tool to determine the energy released in the combustion of foods.

Reproduced from VCAA VCE Chemistry Study Design 2017-2021

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UNIT 4

AOS1

How can the diversity of carbon compounds be explained and categorised?

In this area of study students explore why such a vast range of carbon compounds is possible. They examine the structural features of members of several homologous series of compounds, including some of the simpler structural isomers, and learn how they are represented and named. Students investigate trends in the physical and chemical properties of various organic families of compounds. They study typical reactions of organic families and some of their reaction pathways, and write balanced chemical equations for organic syntheses.

Students learn to deduce or confirm the structure and identity of organic compounds by interpreting data from mass spectrometry, infrared spectroscopy and proton and carbon-13 nuclear magnetic resonance spectroscopy.

Outcome 1 On completion of this unit the student should be able to compare the general structures and reactions of the major organic families of compounds, deduce structures of organic compounds using instrumental analysis data, and design reaction pathways for the synthesis of organic molecules.

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UNIT 4 AOS 1, CHAPTER 7

07

Organic chemistry 7A Structure of organic compounds

7D Chirality

7B Naming of organic compounds

7E Properties of organic compounds

7C Isomers

7F Types of organic reactions

Key knowledge • the carbon atom with reference to valence number, bond strength, stability of carbon bonds with other elements and the formation of isomers (structural and stereoisomers) to explain carbon compound diversity, including identification of chiral centres in optical isomers of simple organic compounds and distinction between cis- and trans- isomers in simple geometric isomers

• structures including molecular, structural and semi-structural formulas of alkanes (including cyclohexane), alkenes, alkynes, benzene, haloalkanes, primary amines, primary amides, alcohols (primary, secondary, tertiary), aldehydes, ketones, carboxylic acids and non-branched esters

• IUPAC systematic naming of organic compounds up to C8 with no more than two functional groups for a molecule, limited to non-cyclic hydrocarbons, haloalkanes, primary amines, alcohols (primary, secondary, tertiary), carboxylic acids and non-branched esters.

• an explanation of trends in physical properties (boiling point, viscosity) and flashpoint with reference to structure and bonding

• organic reactions, including appropriate equations and reagents, for the oxidation of primary and secondary alcohols, substitution reactions of haloalkanes, addition reactions of alkenes, hydrolysis reactions of esters, the condensation reaction between an amine and a carboxylic acid, and the esterification reaction between an alcohol and a carboxylic acid

• the pathways used to synthesise primary haloalkanes, primary alcohols, primary amines, carboxylic acids and esters, including calculations of atom economy and percentage yield of single-step or overall pathway reactions.

Image by Juriaan Wossink/Shutterstock.com

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Chapter 7: OrganiC Chemistry

STRUCTURE OF ORGANIC COMPOUNDS In this lesson we will be unpacking the different types of organic compounds and the structural features of each. In particular, we will look at the functional groups that help to define the type of organic compound. This lesson will provide us with the basics on the structure of organic compounds. This knowledge will be built upon as we progress through the chapter. 7A Structure of organic compounds

7B Naming of organic compounds

7C Isomers

7D Chirality

7E Properties of organic compounds

7F Types of organic reactions

Study design dot points

•

the carbon atom with reference to valence number, bond strength, stability of carbon bonds with other elements and the formation of isomers (structural and stereoisomers) to explain carbon compound diversity, including identification of chiral centres in optical isomers of simple organic compounds and distinction between cis- and trans- isomers in simple geometric isomers

•

structures including molecular, structural and semi-structural formulas of alkanes (including cyclohexane), alkenes, alkynes, benzene, haloalkanes, primary amines, primary amides, alcohols (primary, secondary, tertiary), aldehydes, ketones, carboxylic acids and non-branched esters

Key knowledge units

Carbon bonds

4.1.1.1

Alkanes, alkenes and alkynes

4.1.2.1.1

Haloalkanes

4.1.2.1.2

Primary amines and amides

4.1.2.1.3

Alcohols

4.1.2.1.4

Aldehydes and ketones

4.1.2.1.5

Carboxylic acids

4.1.2.1.6

Esters

4.1.2.1.7

Key terms and definitions • Bond strength how strongly bonds hold atoms together • Bond length length of a bond • Bond energy measurement used to indicate the amount of energy needed to break bonds, measured in kJ/mol • Electronegativity tendency of an atom to attract a pair of bonding electrons • Hydrocarbons organic compounds consisting of carbon and hydrogen • Valence number number of electrons in the outer shell (valence shell) of an atom • Saturated hydrocarbons molecules that have only single carbon-carbon bonds • Unsaturated hydrocarbons molecules that have at least one double or triple carbon-carbon bond • Molecular formula formula that shows the number of atoms of every element in a molecule • Structural formula structural representation of a molecule that shows the atoms in the compound, the number of each atom, and how atoms are arranged and bonded to each other • Semi-structural formula condensed form of a structural formula that does not show all bonds between atoms in a compound • Skeletal structure representation of a molecular structure where covalent bonds are shown as lines. Carbon atoms are shown as vertices and hydrogen atoms bonded to carbon atoms are not shown • Terminal carbon found at the end of the carbon parent chain and is bonded to one other carbon • Alkyl groups groups formed by removing one hydrogen atom from the equivalent alkane chain

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• Functional groups specific groups of atoms within a compound that affect the properties of the compound. Compounds containing the same functional group have similar chemical properties • Primary amine compound with an amino functional group (NH2 ) where the nitrogen atom is only bonded to one carbon atom • Amide compound that has the amide functional group (CON) • Aldehydes compounds that contain an aldehyde functional group (CHO) • Ketones compounds that contain a carbonyl (CO) functional group. The carbon atom in the CO group is attached to 2 alkyl groups • Carboxylic acids compounds that contain a carboxyl functional group (COOH) • Esters compounds that contain an ester functional group (COO) • Haloalkanes alkanes that contain a halogen • Alcohols compounds that contain a hydroxyl functional group (OH) bonded to a carbon atom in the carbon chain • Primary alcohols alcohols where the hydroxyl functional group (OH) is bonded to a carbon with only 1 alkyl group • Secondary alcohols alcohols where the hydroxyl functional group (OH) is bonded to a carbon with 2 alkyl groups • Tertiary alcohols alcohols where the hydroxyl functional group (OH) is bonded to a carbon with 3 alkyl groups

Carbon bonds 4.1.1.1 OVERVIEW

The bonding between carbon and other atoms can have different characteristics. The strength of these bonds depends on the interaction of various factors including bond length and bond energy. THEORY DETAILS

Carbon bonding is the foundation of all organic compounds. As a result, exploring the nature of these bonds provides us with a solid basis to develop an understanding of the different types of organic compounds that exist. Organic compounds are identified as those where carbon is covalently bonded to other atoms. Most often this involves hydrogen, nitrogen, or oxygen. Hydrocarbons, made up of only carbon and hydrogen atoms, are an important set of organic molecules that are predominantly found in naturally occurring fuel sources. The ability of carbon to bond to a variety of different atoms is due to the fact that it has a valence number of four and can therefore form four covalent bonds in a tetrahedral arrangement. The strength of the bonds however may vary depending on the atoms involved. The bond strength of a particular carbon bond depends on the bond energy, measured in kJ/mol. There are various factors that impact the amount of energy required to break a bond, including: • bond length. • the difference in electronegativity of the atoms involved. • size of the atoms. It is important to note that it is the combination of these factors that results in overall bond energy, and therefore we will not be able to obtain a definitive answer if we only look at each factor in isolation. As shown in figure 1, the distance between the nuclei of the two bonding atoms is known as the bond length. The length of the bond will vary depending on the attraction between the nuclei of the bonding atoms and the shared pair of electrons. Also, the size of the atoms involved in the bond also impacts the bond length, where bonds between larger atoms result in longer bond lengths. Looking at the values in table 1, we can identify a relationship between bond length and bond energy.

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Bond length Figure 1 Illustration of how bond length is measured

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Chapter 7: OrganiC Chemistry

Energy kJ/mol

Length

Bond

Energy kJ/mol

Length

Single bonds

Bond

Energy kJ/ mol

Length

Multiple bonds

H—H

432

74

C—N

305

147

C=C

614

134

H—F

565

92

C—O

358

143

C=N

615

127

H—Cl

427

127

C—P

264

187

H—Br

363

141

C—S

259

181

H—I

295

161

C—F

453

133

C—H

413

109

C—Cl

339

177

C—C

347

154

C—Br

276

194

C—Si

301

186

C—I

216

213

C=O

745 (779 in CO2)

123

Stability

Bond

Bond energy

table 1 Bond energies for common bonds

Bond length Figure 2 The relationship between bond length, energy and stability Branched arrangement

In general, the bond energy (and thus the bond strength) increases as the length of the bond decreases. Taking a closer look at the table we can see that there is a relationship between bond energy and electronegativity. As we know, electronegativity increases as we move across a period from left to right, with fluorine being the most electronegative of all elements followed by oxygen, chlorine and nitrogen. Based on the values in the table, we can see that bond energy also follows this trend in descending order. Not only can bond energy indicate how strong a bond is, it also gives us information about the stability of bonds. As summarised in figure 2, bonds with high bond energy are more stable than those with low bond energies. These factors have important implications on the stability of the different types of organic molecules that we will investigate in this lesson.

Alkanes, alkenes and alkynes 4.1.2.1.1 OVERVIEW

Alkanes, alkenes and alkynes are hydrocarbons with different carbon bonding structures.

Linear chain arrangement

Figure 3 Common structural arrangements of alkanes CH 2 CH2

CH2

CH2

CH2 CH 2

Figure 4 Examples of cycloalkanes, cyclohexane (left) cyclobutane (right)

THEORY DETAILS

As we learned in Unit 1, alkanes are saturated hydrocarbons that have only single carbon-carbon bonds in their structures. The general formula of alkanes is CnH2n+2 , where n represents the number of carbon atoms present in the structure. As seen in figure 3, alkanes are usually arranged as linear chains or branched chains (due to the presence of side chains); however, some alkanes can be organised in a cyclic manner. As shown in figure 4, the two terminal carbon atoms in a cycloalkane bond to form a closed ring with no terminal carbon. In this structure, each carbon atom is bonded to two adjacent carbon atoms and two hydrogen atoms. Unlike straight-chain alkanes and branched alkanes, cycloalkanes have the general formula as CnH2n. Benzene is a cyclic compound which has 6 carbons in the ring structure, each bonded to one hydrogen and two carbon atoms. It looks like it is lacking bonds but in fact, one electron from each carbon atom is shared (delocalised), resulting in a very stable formation. Two representations of benzene are shown in figure 7. Alkenes are unsaturated hydrocarbons that have at least one carbon-carbon double bond in their carbon chain. The general formula of alkenes is CnH2n which is similar to the general formula of cycloalkanes however it is important to note that both are structurally quite different and therefore would behave differently. Although we often see and draw alkenes as linear structures, in reality the double bond causes a ‘kink’ in the carbon chain, as shown in figure 5.

Figure 5 Structural formula of but-2-ene, an example of an alkene. The red box indicates the carbon-carbon double bond.

Figure 6 The structural formula of but-2-yne, an example of an alkyne. The red box indicates the carbon-carbon triple bond.

Alkynes are also unsaturated hydrocarbons but, as shown in figure 6, they have at least one carbon-carbon triple bond in their carbon chain. The general formula of alkynes is CnH2n–2. As we know from Unit 1, all of these hydrocarbons have the capacity to be bonded to alkyl and functional groups, a concept that we will work through during the course of this lesson. The organic compounds shown above have been represented as structural formulas. Although this is one of the most common forms of representing the structure of these compounds, there are other ways in which these structures can be represented. Table 2 is a summary table of four formula types.

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Figure 7 Structural representations of benzene

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table 2 Different types of formulas and their representations

Formula molecular formula

Representation C4H8O2

structural formula H

semi-structural (condensed) formula

H H

H

C

C

C

H H

H

O C O

H

CH3CH2CH2COOH or CH3(CH2)2COOH

skeletal structure

O H O

Source: VCAA data book

The first three formula types should be quite familiar as they are often used throughout chemistry books and resources. Skeletal structures however are not as widely used as the first three. With skeletal structures, the main elements to be aware of are: • carbons in the carbon chain are represented as vertices. • hydrogens are generally omitted unless part of a functional group.

δ

δ

C

X

• atoms in a functional group are shown.

Haloalkanes 4.1.2.1.2 OVERVIEW

Haloalkanes are alkane-based compounds which have one or more halogen atoms bonded to the carbon chain.

Figure 8 Representation of a polar bond between a carbon and halogen atom

THEORY DETAILS

Haloalkanes are formed by replacing one or more hydrogen atoms in the carbon chain of alkanes with a halogen; atoms found in group 17 of the periodic table. As halogens are more electronegative than carbons, they have a greater tendency to attract the shared pair of electrons to their own nucleus. This results in a polar bond. As shown in figure 8, the carbon atom has a partial positive charge, while the halogen atom has a partial negative charge, thereby making the bond polar.

(a)

(b)

As shown in figure 9, haloalkanes look similar to their alkane counterpart; however, the polar nature of the carbon-halogen bond causes the molecule to have slightly different properties to its comparable alkane. We will discuss this further in a later lesson. In addition to halogens, organic compounds can also have other functional groups as part Figure 9 (a) Structural formula of of their structures. In the next sections of this lesson, we will investigate functional groups pentane; (b) Structural formula of 2-chloropentane, an example of of common organic compounds. a haloalkane

Primary amines and amides 4.1.2.1.3 OVERVIEW

re m ov e

Amines and amides are compounds whose functional group contain a carbon-nitrogen bond. In this lesson, we will focus on primary amines and amides. THEORY DETAILS

Ammonia

Due to the similarity in structure, amines are thought to be derived from ammonia. As seen in figure 10, a hydrogen atom found in ammonia is removed, resulting in the formation of an amino group (NH2 ). The nitrogen in the amino group is then able to bond to an alkyl group . Therefore, molecules with this functional group are considered as amines. As nitrogen can take part in three bonds, it can potentially bond to three alkyl groups. The number of alkyl groups bonded to the nitrogen atom determines the nature of the amine. Figure 11 shows three types of possible amines that exist; h...