Chem 240. Introduction. Chapter 1. Dr. Seham ALTERARY nd semester

Transcription

1 Chem 240 Chapter 1 Introduction Dr. Seham ALTERARY nd semester

2 Chapter outlines: What is Organic Chemistry? Importance of organic compounds. The Atomic Structure & Electronic configration. Electronegativities of atoms. Chemical Bonding. a) Covalent Bonding. b) Ionic Bonding. i) Polar Covalent Bonding. ii)coordinate Covalent Bonding. Formulas and diagrams. Atomic orbitals and hybridization. Organic Families Function groups.

3 What is organic chemistry? Organic chemistry is a branch of chemistry that involves the study of organic carbon compounds. It is the chemistry of hydrocarbons (C & H) and their derivatives (containing other elements such as O, X& N). Organic chemistry is the chemistry of compounds that contain the element carbon. Carbon compounds are central to life on this planet.

4 Importance of organic compounds Why organic chemistry is important? Because it is the study of life and all of chemical reaction related to life. The chemical substances that make up our bodies; are organic. 1. DNA: the giant molecules that contain all the genetic information for a given species. 2. proteins: blood, muscle, and skin. 3. Enzymes: catalyze the reactions that occur in our bodies. Petroleum: furnish the energy that sustains life. Polymers: Cloths, cars, plastic, kitchen appliances. Medicine.

5 Carbon compounds are called organic because it was originally thought that they could be produced only from living organisms. However, In 1828, Friedrich Wöhler discovered the synthesis of organic compound from inorganic compound. Friedrich Wöhler Urea: is an organic product of protein metabolism in mammals.

6 Though the periodic table has only 118 or so elements there are obviously more substances in nature than 118 pure elements. This is because atoms can react with one another to form new substances called compounds. Formed when two or more atoms chemically bond together. The resulting compound is unique both chemically and physically from its parent atoms. Periodic Table of Elements

7 The Atomic Structure Atoms : are made up of protons, neutrons and electrons. The protons and neutrons are located at the center of the atom; the nucleus. The electrons occupy the space surrounding the nucleus at varying distances. These electrons can be divided into core electrons and valance electron. The valance electrons are outermost & are involved in chemical reactions. Classical atom: electrons in orbit around the nucleus

8 The Atomic Structure 4 He 2 Atomic mass the number of protons and neutrons in an atom. Atomic number the number of protons in an atom.! Note that; number of electrons = number of protons Electrons are arranged in Energy Levels or Shells around the nucleus of an atom.

9 Evaluating Exam 3 mins

10 Electronegativities of atoms : In 1916, the American chemist Gilbert Newton Lewis proposed that chemical bonds are formed between atoms because electrons from the atoms interact with each other. G. N. Lewis -Lewis had observed that many elements are most stable when they contain eight electrons in their valence shell. -He suggested that atoms with fewer than eight valence electrons bond together to share electrons and complete their valence shells.

11 Helium; He, Neon; Ne and Argon; Ar are atoms which do not react with other atoms. They are called the Inert Gases (or Noble Gases) because of this. He Ne Ar Kr Xe Rn

12 Each of these gases has a full outer electron shell (orbit). Examples of three of Inert gases: Atom Electronic configuration Diagram configuration 1) He p2n 2) 20 Ne 2, p10n 3) 40 Ar 18 2,8,8 18p22n

13 Increasing electronegativity Electronegativity Electronegativity is the relative tendency of a bonded atom to attract electrons to itself. Values of Electronegativity of atoms : Increasing electronegativity

14 Chemical Bonding A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. Why do atoms bonds? According to Lewis theory, in interacting with one another atoms can achieve a greater degree of stability by rearrangement of the valence electrons to acquire the outer-shell structure of the closest noble gas in the periodic table.

15 a) Incase of atoms with similar electronegativity values: a)covalent Bonding A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms and other covalent bonds. Example A covalent bond forming H 2 where two hydrogen atoms share the two

16 b) Incase of atoms with different electronegativity values: A polar covalent bond is one in which one atom has a greater attraction for the electrons than the other atom. i) Polar Covalent Bonding δ+ δ- Cl + - H Cl Cl The electron cloud in a σ-bond between two unlike atoms is not uniform and is slightly displaced towards the more electronegative of the two atoms. This causes a permanent state of bond polarization, where the more electronegative atom has a slight negative charge (δ ) eg; Cl, and the other atom has a slight positive charge (δ+) eg; H.

17 In case of covalent bond the two atoms contributed one electron to the electron pair shared between them. There are molecules in which one atom supplies both electrons to another atom in the formation of covalent bond. A covalent bond thus formed is called.. ii)coordinate Covalent Bonding Example When ammonia, :NH 3 reacts with proton, H + to form an ammonium ion, NH 4+, the nitrogen atom in ammonia supplies both electrons to the new bond.

18 b) Incase of atoms with large different electronegativity values: Ionic bonds form from the electrostatic attraction between oppositely charged ions Atoms become ionic by losing or gaining electrons from the atom it is bonding with. b)ionic Bonding Example The positive sodium ion (Na) and the negative chloride ion (Cl) are strongly attracted to each other. + + Na Cl Na + Cl - +

19 An organic compound is one that has carbon; C as the principal element. An inorganic element is any compound that is not an organic compound. Carbon is unique. C C 6 12 It has 4 electrons in its outer shell arranges 1s 2 2s 2 2p 2 It has room for 4 bonds to 4 other atoms.

20 Carbon atom can form multiple bonds, long chains, side chains and cyclic chains. Examples 1- Multiple bonds; double or triple. Ethylene Formaldehyde Formaldimine Acetylene Hydrogen cyanide Acetonitrile

21 Examples 2-long chains 3-side chains 4-cyclic chains

22 How many bonds to an atom? Covalent Number: is the number of covalent bonds that an atom can form with other atoms. The covalence numbers of atoms commonly found in organic compound are listed in the following table. Covalence Number for Typical Elements in Organic Compounds Element Number of Valence electrons Number of electrons in filled valence shell Covalent number H C N O F, Cl, Br, I (halogens) 7 8 1

23 Formulas and diagrams How molecular formulas, electron dot diagrams and structural formulas are related to one another for several simple compounds? A molecular formula tells us what kind of atoms & how many of each kind are present in a particular molecule. Examples 1- Molecular Formula C n H 2n-2 C n H 2n O 2 The molecular formula of alkyne The molecular formula of carboxylic acids

24 Example Given the skeletal structure and assuming that only hydrogen atoms are missing, (a) Draw the correct structural formula and (b) Write the molecular formula of the completed structure. Investigate how many bonds each atom already has, how many extra bonds each atom needs, Place the missing bonds. Put in the missing hydrogens to obtain the correct structure formula. Count the number of atoms of each kind and represent the molecular formula.

25 2- Electron Dot Diagrams (Lewis structure) Electron valance as electron dots Each of the four "sides" of the symbol represents an orbital in the outermost energy level of the atom. The dots show the configuration of the valence electrons of each element. An unpaired electron is an electron that occupies an orbital singly, and they are available for standard covalent bonds.

26 Examples 1) Oxygen 2 Lone paired electrons 2 Unpaired electrons These unpaired electrons might make two single covalent bonds, as is the case in water (H 2 O). Or, they might make one double covalent bond, as the case of magnesium oxide (MgO). 2) Nitrogen 1 Lone paired electrons 3 Unpaired electrons These unpaired electrons might make three single covalent bonds, as is the case in primary amines (-R-NH 3 ). They can make one single bond and one double bonds as in imine ( ). Or, they might make one triple covalent bond, as the case of hydrogen cyanide ( ).

27 For simple molecules; such as: Methane Ammonia Water It is possible to represent both double and triple bonds using lewis dot configuration. Ethene (Ethylene) Ethyne (Acetylene)

28 3- Stractural Formula Structural formula shows how the atoms in a particular molecule are connected or bonded together. The structural formula of a chemical compound is a graphical representation of the molecular structure, showing how the atoms are arranged. Structural formulas are a version of a molecular formulas in which the connectivity of the atoms is implied. See examples of condensed & cyclic formulas textbook Elements of Organic Chemistry page 13, 14 Table 1.3 &1.4

29 Structural Formula The structure formula can be expressed by several ways Expanded structural formula condensed formula Partially Fully Bond line formula (skeltel formula) Ball-stick model

30 Exercise 1 Given the skeletal structure C-C=C-C And assuming that only hydrogen atoms are missing, (a) draw the correct expanded structural formula, (b) condensed structural formula, (c) bond line formula, and, (d) the molecular formula.

31 Exercise 2 Draw the structural formula for the following: (The expanded, fully condensed & skeletal formulas) a) b) d) e) C 2 H 6 C 2 H 4 C 2 H 2 C 6 H 6

32 Atomic Orbitals: Orbital Picture of covalent Bonds We considered the arrangement of electrons within various energy levels, but did not discuss the region in space occupied by electrons. Atomic orbital represents a specific region in space in which the electron is most likely to be found. s orbital The 1s orbital is spherically symmetric and has no nods.

33 The p orbitals consist of two lobes that resembles a solid figure eight 8 or dumbbell and are designed as 2p x, 2p y, and 2p z. p orbitals The orbital axis can be aligned with any one of the x, y, and z. The two lobes of each orbital are separated by a nodel plane through the atom s nucleus and perpendicular to the orbital axis.

34 There are a total of five d orbitals ( d yz, d xz, d xy, d x 2 -y2 and d z2 ) and each orbital can hold two electron. The transition metal series is defined by the progressive filling of the 3d orbitals. d orbitals

35 Molecular Orbital A molecular orbital is formed when two atomic orbitals overlap to generate a bond. The number of molecular orbital equals the number of atomic orbitals from which they drive. Types of Hybridization -These are the three kinds of hybridization that are important in organic chemistry. 1- sp 3 hybridization Type of hybrid Diagram Atomic orbitals used Number of hybrid orbitals formed Geometry sp 3 s, p, p, p 4 tetrahedral

36 2- sp 2 hybridization Type of hybrid Diagram Atomic orbitals used Number of hybrid orbitals formed Geometry sp 2 s, p, p Trigonal planer 3 3- sp hybridization sp s, p 2 linear

37 Valence Shell Electron Pair Repulsion Geometries (VSEPR Model) This model is based on the idea that bond and lone pair in the valence shell of an element repel each other and seek to be as far apart as possible. Configuration Bond Angles Examples Linear 180 BeF 2, HCN Trigonal planer 120 BeF 3, SO 3 Tetrahedral CH 4, SiCl 4

38 Bond lengths & Bond angles of Ethyne, Ethene, and Ethane Note that: the bond length between either C-H or C-C

39 Function groups The Organic Families There are several million organic compounds. The study of their chemistry is made possible only because we are able to classify them into a limited number of families depending on the function group. A function groups is a reactive portion of an organic molecule, an atom, or a group of atoms that confers on the whole molecule its characteristic properties. All compounds with the same function group belongs to one family. Members of a given organic family react in a similar and predictable manner.

40 Hydrocarbons are compounds made up of only the elements carbon and hydrogen. They may be aliphatic or aromatic. Hydrocarbons

41 Compounds containing C=O

42 Compounds containing C-Z σ bonds

43 Functional groups discussion alcohol HO C alkene CH C O ketone O C CH O ester HO C O carboxylic acid

44 carboxylic acid discussion O C H 2 C OH CH ether O CH O C ester O CH 2 C O aldehyde amine NH 2 C H 2 C nitrile N H

45 Exercise 3 Identify the class of compound represented by each structure Answer

46 Orbital hybridization in Alkanes: Atomic orbitals can be combined and reshaped to make other orbitals of different shapes and proparties. There are two basic types of orbitals that can result from such process: 1- Hybrid Orbitals: They result from the combinations of orbitals within a given atom. 2- Molecular Orbitals: They result from the combinations of orbitals between atoms as bonding takes place to form molecule.

47 When atomic orbitals (pure or hybrid) of different atoms overlap to form covalent bonds, they may approach each other in major two ways: head to head, or sideways. Head to head overlap: + s s s-s + s p s-p + p p p-p

48 Sideways overlap: + or p p p-p! Note that: s orbitals has only one possible way to overlap the so called head to head. p orbitals can approach each other either side ways or head to head. When orbitals approch each other in a head to head fashion, the resulting covalent bonds are called sigma bonds.

49 Orbital Hybridization Theory The valance shell of C atom Ground state Excited state sp 3 hybridization Excited state Example: Methane; CH 4 Gives the tetrahydral structure.

50 1- Methane The Three Simplest Alkanes The simplest member of alkane family;ch 4 which has the shape of tetrahydral with an sp 3 hybridization. Sigma bond between s and sp 3 orbitals Bond Angles: HCH-bond angles are o. Bond Length: All C-H bonds are 1.09A o.

51 2- Ethane H H H H C C H H The second member of the alkane series; C 2 H 6. Each two carbons are linked to each other using an sp 3 hybridization since they are single bonds. Note that it is larger than methane by a CH 2 or methylene group. Hybridization Bond angles: HCH bond angle is o. Bond length: C-H bond length is 1.09A o and C-C bond length is 1.54A o. Sigma bond between two sp 3 orbitals

52 3- Propane The third member of the alkane series; C 3 H 8. carbons are linked to each other in a similar way using an sp 3 hybridization since they are single bonds. Note that it is larger than ethane by a CH 2 or methylene group. Hybridization In alkanes of 3 carbon atoms or more, the main chain aquires a zig-zag structure due to the angle between C-C bonds, see propane structure. It can be easily seen than the only type of covalent bonds present in alkanes are sigma bonds, ( single bond)

53 Hybridization in Alkenes Example: For the simplest alkene, Ethylene, or Ethene CH 2 CH 2 2p x1 2p y1 2p z 0 2p x1 2p y1 2p z 1 sp 2 sp 2 sp 2 2p z 1 2s2 2s1 1s 2 1s 2 1s 2 Promotion Hybridization Ground state Excited state Hybridization These orbitals are formed in the following manner: 2s orbital and only two of the three 2p orbitals hybridize, the result is : 3 equivalent sp 2 hybrid orbitals and one unhybridized orbital 2p z orbital.

54 (a) 2s 2p x 2p y 2p z Hybridization The hybridization of 2s, 2p x and 2p y to form sp 2 (b) sp 2 sp 2 2p z sp 2 (or) 120 2p z 2p z 2p z The three sp 2 orbitals get as far away from each other as possible assuming a planer arrangement with an angle of 120 between hybrid orbitals, (trigonal planer). The remaining unhybridized 2p z orbital is perpendicular to the plane of the sp 2 orbitals. 54

55 Bond Formation in Ethylene The two carbons of Ethylene are attached to gather by an Sp Sp 2 = σ bond. The 2p z 2p z overlap resulting the carbon---carbon double bond = π bond. 120 H H 2p z C sp 2 sp 2 2p z 2p z C 2p z H H overlap of sp 2 and p orbitals π σ π H Equivalent to H C C H H 55

56 The Ethylene molecule has trigonal planer geometrical shape, with a bond angle equal to 120, and bond length 1.34 A Note that; the restricted rotation about the carbon carbon double bond and the planar geometry give type of geometric isomersim called The E/Z notation 56

57 Energy Energy Energy Example The Hybridization of Alkynes. For the simplest alkyne, Ethyne,or Actelyne H C C H 2p 1 x 2p 1 2p 0 y z 2p x1 2p y 1 2p z 1 2p y 1 2p z 1 2s 2 2s 1 sp sp 1s 2 1s 2 1s 2 Ground state Excited state sp-hybridized state

58 The hybridization of a 2s orbital and 2p orbital to form two linear sp-hybridized orbitals with bond angle 180 and two unhybridized p orbitals perpendiculy oriented to each other and to the plan of the hybrid sp orbitals. 2s 2p x 2p y 2p z Hybridization The sp orbitals have ½ s character & ½ p character 180 sp 2p z 2p y sp 58

59 a) Bond Formation in Acetylene σ p z H C sp p z P y sp C H P y The C(triple bond)c bond is composed of one sigma bond and two pi bonds. sp-sp P y -p y, and p z -p z π C H C H Head-to-head overlap between two sp-hybrid orbitals to form σ bond between 2 carbons of acetylene and side-side overlap of the p y orbitals and p z to form two π bonds. π

60 The comparative chart of bond length of alkane, alkene and alkyne is given below: Class Alkane Bond C C C H Hybridized bond orbital sp 3 sp 3 sp 3 1s Bond Length 1.54 Å Å Structure Tetrahydral Alkene C C C H sp 2 sp 2 sp 2 1s 1.34 Å Å Trigonal planar Alkyne C C C H sp sp Å sp 1s Å Linear

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