Organic Chemistry L1.pdf

Click to edit Master title style Organic Chemistry D r. A. M. A l - H a i d e r i 1 s t s t a ge 2 nd t e r m 1 Click to edit Master title style...

Click to edit Master title style Organic Chemistry D r. A. M. A l - H a i d e r i 1 s t s t a ge 2 nd t e r m 1 Click to edit Master title style 1.1 Organic Chemistry Organic chemistry is the study of carbon compounds. The word “Organic” describe substance obtained from living sources (plant and animals). This idea was challenged in 1828 by Friedrich Wohler when he heated ammonium cyanate ( a mineral-word ) compound and produced Urea. 2 2 Click to edit Master title style Chemists soon synthesized methane, acetic acid , acetylene and many other organic compounds from inorganic sources. Today, well over 14 million synthetic and natural organic compound are known. This number is significantly greater than the 100,000 or so known inorganic compound. Carbon can from more compound than any other element because C- atoms are able not only to from single, double, and triple C-C bounds, but also to link up with each other in chains and ring structures. The branch of chemistry that deals with carbon compounds is “Organic Chemistry”. Dictionaries define an organic compounds as a compounds of carbon, but that definition would include carbonate, cyanides, cyanate and other carbon-containing ionic compounds usually classified as inorganic. Here is a practical working definition: all organic compounds contain carbon, nearly always bonded to itself and to hydrogen and often to other elements (e.g O,N,X,….etc) as well. 3 3 Click to edit Master title style 1.2 The Structures of Organic Molecules 1.2.1- Lewis pictures of organic molecules The simplest method of communicating about molecules is to draw Lewis structures of them. The bond holding molecules together are covalent, that is they result from sharing of electrons normally a pair of electrons - by two atoms (or electron dot picture). Example: CF4 Q1: draw the Lewis picture and indicate the covalence element of the following: H2, CH4, (CH3)3B, NH3, N2H4 and C2H6 4 4 Click to edit Master title style 1.2.2-Stick picture of organic molecules Rather than the trouble to show each electron - pair being shared, we may use “stick bounds” substituting one stick for each pair of shared electrons. Rather than Rather than 5 5 Click to edit Master title style Q2: use stick pictures to represent the following compounds Borane (BH3), H2O, CH3OH, and C2H6. then indicate the total number of covalent bonds and the number of pairs of unshared electrons. 6 6 Click to editPicture 1.2.3-Lewis Master of title stylewith Multiple Bond Molecules The covalence of elements sometimes satisfied by the sharing of two pairs or three pairs of electrons , forming double or triple bonds. Example: O2 or Example: CH2O or 7 7 Click to edit Master title style Q3: draw electron dot pictures and stick picture of the following compounds: CH2O (formaldehyde), CO2, CO3, C2H2 (acetylene), CN (cyanide ion) the indicate the number of single and multiple bonds present in each case. 8 8 Click 1.2.4 to edit Master Electron title style configuration and the valence bond theory: Electrons spend most of their time at regions about an atomic nucleus , these regions are called orbitals. For the carbon atom and for most organic compounds only S orbitals (spherical) and P orbitals (shaped like opposing teardrops) are important. If we imagine the nucleus to be at the center of three – dimensional coordinate system ,there is only one orientation for the spherical S orbital while p orbitals may be further designated as Px ,Py ,Pz depending on the axis along which they are aligned. Each orbital may contain up to two electrons “a limitation of the pauli exclusion principle“ 9 9 Click We mayto edit something indicate Master title style about the size of the orbitals and the number of electrons it contain by use the spectroscopic notion 1𝑠 2 2𝑠 2 2𝑝2 2𝑝2 2𝑝2 and so forth in which the number before the S or P is the principle quantum number (which increases with size). All orbitals having the some shell principle quantum numbers are said to be in the same shell and those having the largest principle quantum numbers are in the valence shell. It is the valence shell electrons that are shown in electron- dot picture. We distribute the electron according to the Aufbau process, filling them in this order 1s 2s 2p 3s 3p All 2p orbitals in an atom are degenerate (have the same energy) as are all 3p orbitals. So in placing electrons in orbitals one electron is put into each orbital (2px, 2py 2pz ) before putting two into any one orbital (Hund’s rule). In the valence bond theory covalent bonds resulting from the overlap of orbitals, usually , half- full orbitals , from the valence shell of each of the atoms involved. That is the orbitals in an atom that can be used in forming a covalent bond, are normally those valence shell orbitals that contain only one unpaired electron. 1010 Click to show Example: editelectron Master title style configuration of nitrogen, how many covalent bond would we expect to form on the basis of this configuration if bond information involves the sharing of one electron from each atom? Answer: N (7e) 1𝑠 2 2𝑠 2 2𝑝1 2𝑝1 2𝑝1 We would predict the formation of 3 covalent bonds because there are 3 unpaired electrons. Q4: Sketch the orbitals expected to be available for bonding in boron and show electron configuration.is boron has only one covalent bond? Answer : B(5e) 1𝑠 2 2𝑠 2 2𝑝1 2𝑝0 2𝑝0 We predict the formation of one covalent bond because there is one unpaired electron B 2px, orbital sketching is 1111 Click to edit Q5: show Master the electron titleof style configuration carbon (6e) , how many covalent bonds we expected to form, sketch the orbital expected to be available for bonding Answer 5: C(6e) 1𝑠 2 2𝑠 2 2𝑝1 2𝑝1 2𝑝0 Two bonds expected 1.2.5 hybridization of orbitals: In its stable compounds carbon does not exhibit the electron configuration for the free carbon atom (see question 5 above ). Instead, as it bonds ,carbon(also boron) is said to undergo hybridization of its orbitals. In hybridization it "mixes" its S orbital with one or more P orbitals to create a new set of hybrid orbitals with equivalent shapes and energies. In hybrids are designated as sp , 𝑠𝑝2 and 𝑠𝑝3 hybrid orbitals. We can always determine which sort of hybridization carbon can exhibiting in its stable compounds by counting the number of atoms bonded to the carbon-atom: 1212 Click to edit Master title style 4 atoms= 𝒔𝒑𝟑 hybridization 3 atoms= 𝒔𝒑𝟐 hybridization 2 atoms= sp hybridization 1313 Click to edit Master title style When applying valence-bond theory we may show hybridization of carbon by using orbital diagrams: Free C (6e) The lines represent orbitals and the arrows direction of the spin of the electrons. Through hybridization carbon may thus form four bonds rather than two by overlap between orbitals and orbitals from other atoms any unhybridized P orbitals remain as pure P orbitals and also may be used in bonding. 1414 Click to edit Master title style Example: What type of hybridization is exhibited by boron in the BH3 molecule? Answer: hybridization because B bonded to 3-atoms. Q6: what type of hybridization is exhibited by carbon in the following compounds (a) CH4 (B) ethylene (C) acetylene. Q7: consider the compound C2H3F , (fluoro ethane) (a) Draw the Lewis picture (b) what type of hybridization is exhibited by the C-atom. (c) Draw the orbital diagram for one of the carbons. (d) Sketch the set of hybrid orbitals involved , then draw if any "pure" P orbitals remaining? 1515 Click to edit Master title style Q8: consider the molecule C2HF (fluoro ethyne)? (a) Draw the Lewis picture (b) Indicate the type of hybridization exhibited by the C-atoms (c) Draw the orbital diagram for one of the C-atoms (d) Sketch the set of hybrid orbitals involved then draw in any “pure” p orbitals remaining. 1616 Click to edit Master title style Thank You 17