Chiral molecules

Key point: A molecule cannot be chiral if it possesses an improper rotation axis (Sn). Achiral molecule (from the Greek word for ‘hand’) is a molecule that cannot be superim posed on its own mirror image. An actual hand is chiral in the sense that the mirror image of a left hand is a right hand, and the two hands cannot be superimposed. A chiral molecule and its mirror image partner are called enantiomers (from the Greek word for ‘both parts’). Chiral molecules that do not interconvert rapidly between enantiomeric forms are optically active in the sense that they can rotate the plane of polarized light. Enantiomeric pairs of molecules rotate the plane of polarization of light by equal amounts in opposite directions. A molecule with an improper rotation axis, Sn, cannot be chiral. A mirror plane is an S1 axis of improper rotation and a centre of inversion is equivalent to an S2 axis; therefore, molecules with either a mirror plane or a centre of inversion have axes of improper rotation and cannot be chiral. Groups in which Sn is present include D_nh , N_D , and some of the cubic groups (specifically, Td and Oh ). Therefore, molecules such as CH₄ and Ni (CO)₄ that belong to the group Td are not chiral. That a ‘tetrahedral’ carbon atom leads to optical activity (as in CHClFBr) should serve as another reminder that group theory is stricter in its terminology than casual conversation. Thus CHClFBr (11) belongs to the group C1, not to the group Td; it has tetrahedral geometry but not tetrahedral symmetry. When judging chirality, it is important to be alert for axes of improper rotation that might not be immediately apparent. Molecules with neither a centre of inversion nor a mirror plane (and hence with no S1 or S2 axes) are usually chiral, but it is important to verify that a higher-order improper-rotation axis is not also present. For instance, the qua ternary ammonium ion (12) has neither a mirror plane (S₁) nor an inversion centre (S₂), but it does have an S4 axis and so it is not chiral.

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المزيد

الآخبار الصحية

لا تبحث عن الدواء.. هذه الأطعمة كفيلة بإيقاف ألم الرأس

لا تترك زجاجة الماء في السيارة.. احذر شربها

عادة يومية.. كيف يؤثر تصفيف الشعر على جهازك التنفسي؟

احذر هذه الأطعمة.. تسرع الشيخوخة وتفقد بشرتك شبابها

المزيد

الآخبار التكنلوجية

الأوكتان وتأثيره: نصائح لحماية محرك سيارتك بكفاءة وأمان

حرب شهر سبتمبر تشتعل.. ماذا في جعبة أبل وسامسونغ؟

واتساب يطلق ميزة لتعديل الرسائل بالذكاء الاصطناعي

على الكوكب الأحمر.. ناسا تعثر على "قبعة الساحرة"

المزيد

مواضيع ذات صلة

The assignment of molecular symmetry from vibrational spectra

Information from the symmetries of normal modes

The exclusion rule

Molecular vibrations

Polar molecules

Character tables

Symmetry operations, elements and point groups

The stability field of water

Standard potentials and spontaneity

Heterogeneous acid–base reactions

Superacids and superbases

Acidic and neutral solvents