Organic Chemistry, Fifth Edition - JulietHahn.com

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Organic Chemistry, Smith Fifth Edition, modified by Dr. Hahn

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Organic Chemistry

• Organic chemistry is the chemistry of carbon compounds.

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Atomic Structure

• The nucleus contains positively charged protons anduncharged neutrons.

• The electron cloud is composed of negatively chargedelectrons.

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Atomic Structure-1

• The atomic number is the number of protons in thenucleus and also the number of electrons surrounding(i.e., protons = electrons).

• The mass number is the number of protons plusneutrons in the nucleus (e.g., 𝟏𝟏𝟏𝟏

𝟔𝟔𝐂𝐂 has six protons andsix neutrons)

• In a neutral atom, the number of protons equals thenumber of electrons.

• The atomic weight of a particular element is theweighted average of the mass of all its isotopesreported in atomic mass unit (amu).

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In the periodic table

# protons & # electrons 6 atomic number

C # protons + # neutrons 12.01 atomic mass

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Isotopes

• Isotopes are atoms with the same number of protons but a different number of neutrons.

• Mass number is the sum of the protons and neutrons in an atom.

C126 6C

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Ions

• A cation is positively charged and has fewerelectrons than protons.

• An anion is negatively charged and has moreelectrons than protons.

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The Periodic Table• Elements in the same row are similar in size. (period)• Elements in the same column have similar electronic

and chemical properties. (group)

Figure 1.1

period #(row)

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Electronic Configurations• The aufbau principle states we must fill the lowest energy

orbitals first.• Hund’s rule states that when there are two or more orbitals of the

same energy (degenerate), electrons will go into different orbitals rather than pairing up in the same orbital.

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Atomic Orbitals• An s orbital has a sphere of electron density and is lower

in energy than the other orbitals of the same shell.• A p orbital has a dumbbell shape and contains a node

(no electron density) at the nucleus. It is higher in energythan an s orbital.

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electron configurations• Every move to right (in periodic table) – adds

one outer most electron• Last electron added is located by (electron

configuration symbol) period number and s,p,d block

• Group number is same as # of valence electrons

• Organic Chemistry – mostly main group elements (groups 1A, 2A, 3A,4A,5A,6A,7A, 8A)

• Organic Chemistry – mostly C,H,N,O

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Electron configuration of some elements (be able to do for most

main group above period 6)C

Ca

N

Br

1s2 2s2 2p2

1s2 2s2 2p6 3s2 3p6 4s2

1s2 2s2 2p3

1s2 2s2 2p2 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5

End class 8/9/17

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valence electrons = electrons in highest period number=group #

(underlined are valence electrons)

C group 4 = 4 valence e

Ca group 2 = 2 valence e

N group 5 = 5 valence e

Br group 7 = 7 valence e

1s2 2s2 2p2

1s2 2s2 2p6 3s2 3p6 4s2

1s2 2s2 2p3

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5

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Bonding• Bonding is the joining of two atoms in a stable

arrangement.

• Atoms can form either ionic or covalent bondsto attain a complete outer shell (octet rule forsecond row elements).

• Ionic bonds result from the transfer of electrons fromone element to another. (selfish)

• Covalent bonds result from the sharing of electronsbetween two nuclei. (friendly and cooperative)

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Ionic Bonding• An ionic bond generally occurs when elements on the

far left side of the periodic table combine with elementson the far right side, ignoring noble gases.

• A positively charged cation formed from the element onthe left side attracts a negatively charged anion formedfrom the element on the right side (e.g., sodiumchloride, NaCl).

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Ionic Bonding-1• Li loses its one electron to make 𝐋𝐋𝐋𝐋+ which has no

electrons in second shell. However, it has a completefirst shell.

• F gains one electron to make 𝐅𝐅− which has a filledvalence shell (an octet of electrons), like neon.

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Covalent Bonding

• Covalent bonding occurs with elements like carbon inthe middle of the table (e.g., 𝐂𝐂𝐂𝐂𝟒𝟒) with elements thathave similar electronegativity.

• Covalent bonds also occur between two of theelements from the same side of the periodic table (e.g.,𝐂𝐂𝟏𝟏, 𝐂𝐂𝐂𝐂𝟏𝟏).

• A covalent bond is a two-electron bond, and acompound with covalent bonds is called a molecule(covalent molecule).

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Covalent Bonding-1Bonding in Molecular Hydrogen (𝐂𝐂𝟏𝟏)• Hydrogen forms one covalent bond.

• When two hydrogen atoms are joined in a bond, eachhas a filled valence shell of two electrons.

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Covalent Bonding• Electrons are shared between the atoms to complete the

octet.• When the electrons are shared evenly, the bond is said to

be nonpolar covalent, or pure covalent.• When electrons are not shared evenly between the

atoms, the resulting bond will be polar covalent.• F is the most electronegative element. (to determine

polarity of a bond) (memorize the red part of this line)

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Lewis StructuresLewis structures are electron dot representations for molecules.

General rules for drawing Lewis structures:1. Draw only the valence electrons.2. Give every second-row element no more than eight

electrons.3. Give each hydrogen two electrons.

A solid line indicates a two-electron covalent bond.

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How to Draw a Lewis StructureStep [1] Arrange atoms next to each other that you think are bonded together.

• Always place hydrogen and halogens on the periphery because they form only one bond each.

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How to Draw a Lewis Structure-1Step [2] Count the electrons.

• Count the number of valence electrons from all atoms.• Add one electron for each negative charge.• Subtract one electron for each positive charge.• This gives the total number of electrons that must be

used in drawing the Lewis structure.

Step [3] Arrange the electrons around the atoms.• Place a bond between every two atoms, giving two

electrons to each H and no more than eight to anysecond-row atom.

• Use all remaining electrons to fill octets with lonepairs.

• If all valence electrons are used and an atoms does nothave an octet, form multiple bonds.

Step [4] Assign formal charges to all atoms.

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Lewis StructuresCH4 NH3

H2O Cl2

Carbon: 4 e4 H@1 e ea: 4 e

8 e

Nitrogen: 5 e3 H@1 e ea: 3 e

8 e

Oxygen: 6 e2 H@1 e ea: 2 e

8 e 2 Cl@7 e ea: 14 e

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Nonbonding Electrons

• Also called lone pairs• Nonbonding electrons are valence-shell electrons

that are not shared between atoms.

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Multiple Bonds

• If all valence electrons are used and an atom does not have anoctet, form multiple bonds.

• To give both C’s an octet, change one lone pair into onebonding pair between the two C’s, forming a double bond.

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Formal Charge

• Formal charge is the charge assigned to individual atoms in aLewis structure.

• Formal charge is calculated as follows:

• The number of electrons “owned” by an atom is determinedby its number of bonds and lone pairs.

• An atom “owns” all of its unshared electrons and half of itsshared electrons.

End class 8/11/17