Preview Advanced Organic Chemistry (Structure Mechanisms) by Ashutosh Kar (2017)

Preview Advanced Organic Chemistry (Structure Mechanisms) by Ashutosh Kar (2017) Preview Advanced Organic Chemistry (Structure Mechanisms) by Ashutosh Kar (2017) Preview Advanced Organic Chemistry (Structure Mechanisms) by Ashutosh Kar (2017) Preview Advanced Organic Chemistry (Structure Mechanisms) by Ashutosh Kar (2017) Preview Advanced Organic Chemistry (Structure Mechanisms) by Ashutosh Kar (2017)

Advanced Organic Chemistry (Structure & Mechanisms) i Other Related Titles in Chemistry Handbook of Chemical Industries P Vishwanathan Chemical Thermodynamics P Vishwanathan Chemistry for Chemical Industries P Vishwanathan Introduction to Polymer Science and Technology D.K Sharma Textbook of Physical Chemistry (Part 2nd & 3rd) J Lahiri Advanced Organic Chemistry Ashutosh Kar Practical Manual of Biochemistry Sadhana Sharma, Reema Sharma Concept of Physical Chemistry Jyotirmay Lahiri Textbook of Pericyclic Reactions K.C Majumdar, Paritosh Biswas Textbook of Engineering Chemistry Deepa Sharma Organic Chemistry Gene A Davis, Ph D Organic Synthesis Through Disconnection Approach (2nd Edition) P.S Kalsi Engineering Chemistry Friedrich Emich Water Chemistry Mark M Benjamin Textbook of Chemical Engineering — Vol Vikas Zaveri Textbook of Chemical Engineering — Vol P Vishwanathan Advanced Organic Chemistry ■ ■ ■ ■ ■ (Structure & Mechanisms) - • : ' ■ [For B.Sc (Hons.), M.Sc, and Research Scholars] Ashutosh Kar Professor Emeritus, Lingaya's University, Faridabad (India) Formerly Professor, School of Pharmacy, Addis Ababa University, Addis Ababa (Ethiopia) Dean, Chairman & Professor, Faculty of Pharmaceutical Sciences, Guru Jambheshwar University, Hisar (India) Professor, School of Pharmacy, Al-Arab Medical University, Benghazi (Libya) Professor & Head, Department of Pharmaceutical Chemistry, Faculty of Pharmaceutical Science, University of Nigeria, Nsukka (Nigeria) Professor & Head, Department of Pharmaceutical Science, College of Pharmacy, Delhi University, New Delhi m MEDTECH Engaging Sciences—Developing Minds! "QUOTES" "One 3mpaxtant 3ieu to- Succeed 3s Sety-Confidence On Smpoxtant 3Ceu to- Self-Cenfidence 3s 3>iepevuUion" — A R T H U R ASLIE "She 3utwte tBdomp to- Jhase— Who- Relieve in the {Beauty, of, Jhevc Steams." —EIEAINOR ROOSEVEIT "2)*eams WM Only 3tetp %oa actualize %ewt QOOIA Started 3)xeantina When was a QMd {Became a Scientist {Because IDteamt" —APJ Abdul KAIAM TCttotuiedqe 'SeqeU 24faie*te(f, V Jo ^nu HAouzd 'Jo Otbxi JV{z Butfd-uf> J\f[ij (Dux ^ / , J^auqhtsxi-in-Laur ■ Presence of an array of appreciable differences with regard to bonding in ethene vis-a-vis methane; >■ sp2-Hybrid orbitals in ethene as well as all akenes possess a major percentage of the 's' character; and hence, the electrons remain quite closer to the C-nucleus; >> Besides, the respective alteration in the hybridisation pattern brings forth a significant change of the bond angle; >■ Both in ethene and around the sp -hybrid C-atoms of the alkenes the ensuing bond angles stand at 120°—that eventually gives rise to the so-called a flat-structure; and >• Ultimately, the prevalent 'geometrical structural arrangement' indicates explicitly that the various substituents located strategically around the alkene C-atoms usually are as far away from one another as deemed possible.* * This feature also holds good for methane as well 80 ADVANCED ORGANIC CHEMISTRY Formation of are-Bond:Let us look into the fate of the /i-orbitals that failed to take active role in the critical formation of sp -hybrid orbitals In fact, they remain very much persistent as the /»-orbitals wherein we have: >- remain attached to one at each C-atom; and >- each p-orbital comprises one electron only The combination of the above two forms a 'bond'—usually known as the 're-bond' occurring prominently between the respective C-atoms {i.e., the mode of overlap is found to 'sideways on') Formation of a o-Bond: The particular 'end-on-overlap' that eventually gives rise to the formation of the o-bonds In the present foregoing discussion critically involves the so-called 'CHbonds' present both in ethene and methane, plus the other bond existing between the 'C—C bonds' in ethene , bonds p-Orbital bonds bonds (a) bonds Hbonds H (b) bonds bonds bonds bonds (c) Fig 2.8: (a) Orbital structure of Ethene; {b) Structure of Ethene; and (c) Structure of Ethene showing overlap of p-orbitals of C-Atoms Forming rc-Bond Figure 2.8 illustrates the sp2-hybrid orbitals showing one C—C re bond and two C—H re bonds Remarks: These essentially include: The crucial presence of there-bondpredominantly refers to the characteristic features of an 'alkene' which figure it out distinctly from the corresponding 'alkane' The respective 71-bond based upon its typical nature of its sideways overlap of the constituent p-orbitals is found to be definitely weaker than the o-bond Besides, the inherent electrons of there-bondare exposed duly both above and below the plane of the 'alkene' Furthermore, these electrons serve as the major source of reactivity for the alkene towards the 'electrophiles' in particular, such as: 'electrophilic addition of bromine' There-bondpresent in ethene remains significantly strong so as to prevent the rotation around the C—C sigma (o) bond However, it endorses the well-established property of the so-called C—C bond present in ethene Bonding between sp2-hybrid C-atoms of an alkene essentially comprises '1' o-bond and '1're-bond,known collectively as the double bond 81 STEREOCHEMISTRY 2.6.1.3 Hybridisation: Ethyne Let us take into consideration the triple bond (or acetylene bond) present in ethylene, as depicted in Fig 2.9 (a), even though its ensuing stereochemical significance is confined significantly As illustrated in Fig 2.9 (b) the C-atoms in ethylene molecule critically make use of: • one a-orbital, and • one Jt-orbital to give rise to the formation of two equivalent sp-hybrid orbitals Thus, it predominantly leaves two so-called unhybridized rc-orbitals [Fig 2.9 {b)] In this manner, the sp-hybrid orbitals are duly utilized to form the o-bonds: • one to the other C-atom, and • one to the H-atom Furthermore, thep-orbitals located at each C-atom eventually yield two altogether separate and independent ft-bonds, which are located strategically at right angles to each other as may be viewed along the axis of the ethyne molecule [see Fig 2.9 (c)] In usual practice, such n-bonds are termed as 'orthogonal' However, the C-atoms are duly joined with a triple bond (or acetylenic linkage), that eventually causes a further shortening of the usual bond length; and therefore, an enhancement in the overall 'bond energy' Remarks: In nitrites (—C=N) the bonding is almost similar to the triple bond (as seen in acetylene/ethyne)—that is made up of o-bond and orthogonal n-bonds Fig 2.9: (a p-Orbital 81 Fig 2.9: (a Fig 2.9: (a 81 81 81 81 81 81 Fig 2.9: (a 81 Fig 2.9: (a Fig 2.9: (a 81 (b) (c) 81 Fig 2.9: (a) Structure of ethyne; (b) Orbital structure of ethyne, (c) View along the axis of the ethyne molecule There is an apparent increase in the actual percentage of the 's' character related to the particular C-hybrid orbitals invariably found along the series stated below: • ethane [carbons sp3, tetrahedral]; • ethene [carbons sp2, trigonal]; and • ethyne [carbons sp, linear] 82 ADVANCED ORGANIC CHEMISTRY It obviously implies a greater electron density profile located specifically at the C-nucleus as we move along the above series; and this is incomplete agreement with respect to the observed shorter C—C bond lengths in a progressive manner as could be seen in the following average values: >■ ethane : 0.154 nm; >- ethene : 0.133 nm; and >- ethyne : 0.12 nm NOTE: Since the crucial presence of the 'radial distribution of the ensuing electron density' the observed rotation around the tripple bond is expected to be absolutely 'free' However, it never changes the so-called 'original shape of the molecule' at all 2.6.2 Conformation Preamble: The term 'conformation' refers to the spatial arrangements of various atoms thereby affording the distinction between the 'stereoisomers' which may be interconverted by rotations about the single bonds perceptively Following are the various important aspects of the phenomenon of conformation, such as: -I Conformation: Ethane; □ Conformation: Butane; □ Chair Conformation: Cyclohexane; and □ Boat Conformation: Cyclohexane, which shall now be discussed individually in the sections that follows: 2.6.2.1 Conformation: Ethane In order to expatiate the dihedral angles in ethane (C2H6), one should make an attempt to define explicitly the ensuing relationship existing between: • C—H bonds on one C-atom, and • those present on the other C-atom, and to accomplish this objective one may have to view the said molecule in a Newman projection formula (as described in section 4.5.4) In fact, the Newman projection refers to a kind of planar projection very much along one specific bond, that we usually term as the 'projected bond' Figure 2.10 {a, b and c) provides an in-depth view of the ethane molecule within the Newman projection vividly along the C—C bond Nevertheless, in this projection, one may visualise the C—C bond as the projected bond In addition, the Fig 2.10 illustrates the so-called Newman projection (with '6'—as the dihedral angle) that could be attained via two distinct modalities, such as: • ball-and-stick models, and • line-and-wedge formulas Part 'a': Ethane molecule being viewed from the end of the bond a person (viewer) wishes to project; 83 STEREOCHEMISTRY Part ' ' : Resulting end on view as depicted clearly; and Part ' c ' : Representation as the Newman projection in the plane of the page (or board) Ball-arid Stick Models Fig 2.10: Fig 2.10: Fig 2.10: Fig 2.10: Fig 2.10: HFig 2.10: H Fig 2.10: Fig 2.10: Fig 2.10: (c) Newman projection (o = Dihedral angle) C—C^ A \ Fig 2.10: H H H (a) Viewing a model of ethene from one end (b) End-on-view Fig 2.10: (a, b and c): Representation of a Newman projection of ethane (C2H6) using: top—the Ball-and-stick models; and bottom—the Line- and Wedge formulas Interestingly, in the so-called Newman projection, the bonds are usually drawn to the centre of the circle are attached to the very C-atom located closer to the observer; whereas, the bonds drawn specifically to the periphery of the circle (dark) are duly linked to the C-atom located further from the observer However, the 'projected bond' (i.e., the C—C bond) is almost hidden Newman projection of ethane (Fig: 2.10 c): If one focuses attention upon the Newman projection of 'ethane' in the above cited figure, one would certainly observe that the three C—H bonds drawn right up to the centre of the circle designate the bonds to the front C-atom particularly Thus, the three C—H bonds drawn to the respective periphery of the circle represent the bonds to the rear C-atom specifically NOTE: However, the Newman projected bond itself, that being the fourth bond attached to each C-atom is hidden Points to Ponder: These essentially include: Newman projection of ethane indeed helps to render it a lot easier and convenient to visualize the critical presence of the so-called dihedral angles existing between its inherent C—H bonds Obviously, the overall specification pertaining to all the dihedral angles in a particular molecule, justifiably specifies its conformation ... Textbook of Physical Chemistry (Part 2nd & 3rd) J Lahiri Advanced Organic Chemistry Ashutosh Kar Practical Manual of Biochemistry Sadhana Sharma, Reema Sharma Concept of Physical Chemistry Jyotirmay.. .Advanced Organic Chemistry (Structure & Mechanisms) i Other Related Titles in Chemistry Handbook of Chemical Industries P Vishwanathan Chemical Thermodynamics P Vishwanathan Chemistry. .. Engineering — Vol P Vishwanathan Advanced Organic Chemistry ■ ■ ■ ■ ■ (Structure & Mechanisms) - • : ' ■ [For B.Sc (Hons.), M.Sc, and Research Scholars] Ashutosh Kar Professor Emeritus, Lingaya's