Chapter 2: Introduction to Organic Chemistry II PDF

College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ Chapter 2: Introduction to Organic Chemistry II Chemistry Department CHEM 1450 College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ After this lecture, the student will be able to: Define Forces among & inside molecules Illustrate the different types of bonds (intramolecular): o Metal...

College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ Chapter 2: Introduction to Organic Chemistry II Chemistry Department CHEM 1450 College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ After this lecture, the student will be able to: Define Forces among & inside molecules Illustrate the different types of bonds (intramolecular): o Metallic o Ionic o Covalent o Coordinate covalent Illustrate the different types of intermolecular forces College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ Intermolecular Forces Are attractive forces between molecules } } Intramolecular Forces Are attractive forces hold atoms together inside a molecule College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ Forces among & inside molecules } There are 2 types of attraction in molecules: intramolecular bonds & intermolecular forces } Intramolecular bonds (ionic, covalent (Polar and non-polar), coordinate, metallic) } Intermolecular forces (IMF) have to do with the attraction between molecules (versus intramolecular forces: the attraction between atoms in a molecule, metal, or compound) } The three IMFs are: 1) H-bonding, 2) dipole - dipole, and 3) London forces order of decreasing strength) (in College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ Generally, intermolecular forces are much weaker than intramolecular forces College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ } Intramolecular Forces includes types of bonding inside the molecule:     Metallic Bond Coordinate Bond Ionic Bond Covalent Bond College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ Metallic bonds are the forces of attraction between the free-floating valence electrons and the positively charged metal ions. These bonds hold metals together } College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ } Sodium chloride, or table salt, is an ionic compound consisting of sodium cations and chloride anions An ionic compound is a compound composed of cations and anions Ionic Bond: The electrostatic forces that hold ions together in ionic compounds, which are formed by the transfer of one or more electrons from one atom to another College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ } } When sodium and chlorine react to form a compound, the sodium atom transfers its one valence electron to the chlorine atom Sodium and chlorine atoms combine in a one-to-one ratio, and both ions have stable octets Metal + Non-metal = Ionic Compound College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ Mg Mg2+ + 2e- 2e- + 2 F Mg + 2F 2F - MgF2 College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ Covalent bond is a chemical bond in which two or more electrons are shared by two atoms } } } If the atoms share 2 electrons a single covalent bond is formed If the atoms share 4 electrons a double covalent bond is formed If the atoms share 6 electrons a triple covalent bond is formed College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ Lewis structure of F 2 F F 7e- 7e- F F 8e- 8e- single covalent bond lone pairs lone pairs F F F F lone pairs lone pairs single covalent bond Lewis structure of water College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ H + O + H single covalent bonds H O H or H O H 2e- 8e- 2e- Double bond – two atoms share two pairs of electrons O C O or O O C double bonds 8e- 8e- 8e- double bonds Triple bond – two atoms share three pairs of electrons N N 8e- 8e- triple bond or N N triple bond 13 College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ } } When two atoms bond their DIFFERENCE in electronegativity determines the covalent bond type Occurs between non metals Non Polar Covalent Bonds Polar Covalent Bonds Non Polar Covalent Bonds College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬  When electrons are shared equally  Has almost no electronegativity difference (0.0 to 0.4)  H-C Bond is non-polar, also any diatomic molecule with identical atoms is nonpolar (eg: O2, F2,N2) College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ Polar Covalent Bonds  Electrons are unequally shared  The greater the difference in electronegativity The more polar is the bond College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ Describing Polar Covalent Bonds δ+ δ– H—Cl The plus sign shows that hydrogen has acquired a slightly positive charge. The minus sign shows that chlorine has a slightly negative charge.  The polar nature of the bond may also be represented by an arrow pointing to the more electronegative atom H—Cl College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ Describing Polar Covalent Bonds College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ Coordinate Bond A coordinate covalent bond, also known as a dative bond or coordinate bond is a kind of 2center, 2-electron covalent bond in which the two electrons derive from the same atom. The bonding of metal ions to ligands involves this kind of interaction. College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ } Intermolecular Forces (IMF): (between molecules)  Hydrogen Bonding  Dipole-dipole interaction  London forces have seen that molecules can have a separation of charge } This happens in polar bonds (the College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ } We + – H Cl greater the ΔEN, the greater the dipoles) Molecules are attracted to each other in a compound by these positive and negative forces + – + – College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ } H-bonding is a special type of dipole dipole attraction that is very strong } It occurs when F, O, or N are bonded to H } They are given a special name (Hbonding) because compounds containing these forces are important in biological systems College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ } Non-polar molecules do not have dipoles like polar molecules. How, then, can non-polar compounds form solids or liquids? } London forces (also called van der Waal forces) are due to small dipoles that exist in non-polar molecules } Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical } The resulting tiny dipoles cause attractions between atoms/molecules (the greater the mass, the greater the London forces) College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ London forces London forces Are present in all compounds Can occur between atoms or molecules Are transient in nature (dipole-dipole are more permanent). London forces are the weakest IMF. College of Medicine – ‫ﻛﻠﯾﺔ اﻟطب‬ } The bigger the IMF’s, the more energy it takes to overcome them...... thus ◦ The bigger the IMF’s, the higher the melting and boiling point } Bonds are stronger than IMF’s, so the melting and boiling points of covalent network solids (i.e.. diamond), ionic compounds (i.e.. salts), and metals (i.e.. iron) are much higher than molecular substances (i.e.. water)