Structural isomers Structural isomers (constitutional isomers): Compounds with the same molecular formulas but different arrangements of the atoms. Example: Draw the structural isomers for C 4 H

CH 3 CH 2 CH 2 CH 3 butane 2402

CH 3 CH 2 CH 2 CH 3 butane CH 3 CHCH 3 2-methylpropane CH 3 (the 2 is redundant in this name) 2403

Example: Draw the structural isomers for C 5 H

Example: Draw the structural isomers for C 5 H 12 CH 3 CH 2 CH 2 CH 2 CH 3 pentane 2405

Example: Draw the structural isomers for C 5 H 12 CH 3 CH 2 CH 2 CH 2 CH 3 pentane CH 3 CH 2 CHCH 3 2-methylbutane CH 3 (2 is redundant) 2406

Example: Draw the structural isomers for C 5 H 12 CH 3 CH 2 CH 2 CH 2 CH 3 pentane CH 3 CH 2 CHCH 3 2-methylbutane CH 3 (2 is redundant) CH 3 CH 3 CCH 3 2,2-dimethylpropane CH 3 (each 2 is redundant) 2407

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Example: Draw the structural isomers for C 2 H 6 O 2410

Example: Draw the structural isomers for C 2 H 6 O CH 3 CH 2 OH ethanol 2411

Example: Draw the structural isomers for C 2 H 6 O CH 3 CH 2 OH ethanol CH 3 OCH 3 methoxymethane (dimethyl ether) 2412

Exercise: Draw and name all the structural isomers for C 6 H 14 (Answer there are 5). 2413

Exercise: Draw and name all the structural isomers for C 6 H 14 (Answer there are 5). The number of structural isomers increases significantly as the number of carbon atoms increases. For example, C 20 H 42 has 366,319 isomers. 2414

Number of carbons Number of isomers for alkanes , ,111,846, ,491,178,805,

Stereoisomerism 2416

Stereoisomerism Stereoisomerism: Isomers having the same molecular formula and the same atom-to- atom bonding, but the atoms differ in their arrangement in space. 2417

Stereoisomerism Stereoisomerism: Isomers having the same molecular formula and the same atom-to- atom bonding, but the atoms differ in their arrangement in space. Geometric isomers: Isomers having the same atom-to-atom bonding, but the atoms differ in their arrangement in space. 2418

Examples: The trans and cis isomers of 1,2-dichloroethene. 2419

Examples: The trans and cis isomers of 1,2-dichloroethene. trans-1,2-dichloroethene 2420

Examples: The trans and cis isomers of 1,2-dichloroethene. trans-1,2-dichloroethene cis-1,2-dichloroethene 2421

Examples: The trans and cis isomers of 1,2-dichloroethene. trans-1,2-dichloroethene (b.p. 48 o C, m.p. -50 o C) cis-1,2-dichloroethene (b.p. 60 o C, m.p. -80 o C) 2422

An example from inorganic chemistry. NH 3 Cl NH 3 Cl Pt Pt NH 3 Cl Cl NH 3 cis isomer trans isomer 2423

An example from inorganic chemistry. NH 3 Cl NH 3 Cl Pt Pt NH 3 Cl Cl NH 3 cis isomer trans isomer common name: cisplatin 2424

An example from inorganic chemistry. NH 3 Cl NH 3 Cl Pt Pt NH 3 Cl Cl NH 3 cis isomer trans isomer common name: cisplatin Only the cis isomer is an effective chemotherapy agent. 2425

Optical Isomers - Chirality 2426

Optical Isomers - Chirality Polarized Light: Plane polarized light consists of electromagnetic waves with the electric component vibrating in one direction. 2427

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Optical Isomer: An isomer that causes rotation of the plane of polarization of light when passed through the substance. 2429

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Chiral (sounds like ki ral): An object that cannot be superimposed on its mirror image is called chiral. 2431

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mirror plane 2434

mirror plane Can superimpose these two molecules; trichloromethane is achiral. 2435

mirror plane 2436

mirror plane Cannot superimpose these two molecules; bromochlorofluoromethane is chiral. 2437

Enantiomers: A chiral molecule and its non- superimposable mirror image are called enantiomers. 2438

Enantiomers: A chiral molecule and its non- superimposable mirror image are called enantiomers. The simplest case is a tetrahedral carbon bonded to four different groups. 2439

Enantiomers: A chiral molecule and its non- superimposable mirror image are called enantiomers. The simplest case is a tetrahedral carbon bonded to four different groups. Chiral molecules lack molecular symmetry. 2440

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Lactic acid has optical isomers. 2442

Naming chiral centers The R, S convention 2443

Naming chiral centers The R, S convention R, S system. A system for specifying the absolute configuration of a chiral center was developed by Cahn, Ingold, and Prelog and is named after them. It is also referred to as the R, S system. 2444

Naming chiral centers The R, S convention R, S system. A system for specifying the absolute configuration of a chiral center was developed by Cahn, Ingold, and Prelog and is named after them. It is also referred to as the R, S system. The orientation of the atoms/groups attached to a chiral center are assigned using a set of priority rules. In simplified form the rules are as follows. 2445

Priority rules 1. Each atom bonded to the chiral center is assigned a priority number on the basis of increasing atomic number, e.g. 2446

Priority rules 1. Each atom bonded to the chiral center is assigned a priority number on the basis of increasing atomic number, e.g. increasing priority H CH 3 NH 2 OH F Cl

2. If the priority cannot be assigned on the basis of the first atom in the group attached to the chiral center, examine the next atom in the group, and so on, e. g. increasing priority CH 2 H CH 2 CH 3 CH 2 OH

2. If the priority cannot be assigned on the basis of the first atom in the group attached to the chiral center, examine the next atom in the group, and so on, e. g. increasing priority CH 2 H CH 2 CH 3 CH 2 OH Note that priority is assigned on the basis of the first point of difference, not on the size of the group attached. For example, CH 2 Cl has higher priority than CH 2 CH 2 CH 2 CH 2 CH

Assigning R and S to a chiral center 1. For each of the four atoms/groups attached to the chiral center, assign a priority order from highest (1) to lowest (4) using the priority order from the previous slides. 2450

Assigning R and S to a chiral center 1. For each of the four atoms/groups attached to the chiral center, assign a priority order from highest (1) to lowest (4) using the priority order from the previous slides. 2. Orient the molecule so that the atom/group with lowest priority (4) is directed away from you, and the three other atoms/groups project towards you. 2451

Assigning R and S to a chiral center 3. If the three atoms/groups facing you have a priority assignment order that increases clockwise the configuration of the chiral center is assigned as R. If the assignment of priority order increases anticlockwise, the configuration of the chiral center is assigned as S. 2452

Assigning R and S to a chiral center 3. If the three atoms/groups facing you have a priority assignment order that increases clockwise the configuration of the chiral center is assigned as R. If the assignment of priority order increases anticlockwise, the configuration of the chiral center is assigned as S. R S

Example: Give the name of the following 2454

Example: Give the name of the following increasing priority H F Cl Br Priority order:

The three top priority atoms attached to the chiral center go in clockwise order, so the name of the compound is: (R)-bromochlorofluoromethane 2456 H (4) Br (1) Cl (2) (3) F

Example: Give the name of the following CH

Example: Give the name of the following CH 3 increasing priority H C O F Priority order:

The three top priority atoms attached to the chiral center go in anticlockwise order, so the name of the compound is: 2459 H (4) OH (2) (1) F CH 3 (3)

The three top priority atoms attached to the chiral center go in anticlockwise order, so the name of the compound is: (S)-1-fluoroethanol 2460 H (4) OH (2) (1) F CH 3 (3)